The following was written by Some editing, layout work, teensy weensy rewriting, and conversion to HTML was subsequently done by me. Also some comments. Without further ado, here's ... Johnny? No... Gotta be Buck!
Here is a list of non-XXVc material that would be easily used with XXVc in the original campaign world, to expand that or create a new one.

Other Game Materials Easily Used To Enhance and Expand "Buck Rogers in the XXVc Century"

Lighthouse, sourcebook for the Alternity campaign setting Star*Drive, TSR
The Lighthouse is a huge, mobile space station, home to diplomats, soldiers and merchants of every type from several governments. It is constructed in the "standard" stacked vertical design which makes it XXVc friendly. It is detailed enough to allow its use in an XXVc campaign with just swapping governments and/or corporations.

It could be placed anywhere in the solar system, could wander around, or might hang within the uppermost atmosphere layer of either Jupiter or Saturn. (A good use for it would be as a semi-"rogue" bottle habitat, perhaps owned or licensed by the Black Brotherhood as a sort of neutral meeting-ground between various movers and players. You know, starting to think of it, it sounds like the concept for the Babylon 5 space station, doesn't it?)

Tales of the Comet, TSR
Using the AD&D 2nd Edition rules, this supplement details aliens, cyborgs, and equipment. Best of all, you can just plop them directly into your BR campaign as well, as the d20 mechanic rules it all!

Gamma World, 4th edition, TSR
Easily handles gennie creation using mutant creation rules. (However, so does the expansion "No Humans Allowed". I'd suggest using "No Humans Allowed", if it is available, and rather use the mutant creation rules for inspiration for new gennies.)

Provides ravaged Earth background material, rules are nearly identical. (Again, there's already a BR supplement that covers Terra and the state she's in in BR - the name escapes me at the moment - but if it is available, I suggest using the "official" supplement to avoid major reworking.) Weapons, technology, organizations, encounters. Add Esper class for psionics. (Psionics is covered under the Oberon Sidhe gennie, with stats given in "No Humans Allowed". It's taken from another supplement, but I don't remember which. Since I have "No Hummans Allowed", I tend to use it as a baseline. The gist of it is that all psionically giftet people are gennies from Oberon - fantastically intelligent, but physiologically crippled... more or less, at any rate. I suggest keeping Espers restricted to Oberon Sidhe, to keep it 'special'.) Older editions vary in rules compatibility.

Treasures of the Ancients, TSR for Gamma World 4th ed.
Provides equipment, weapons and robots. Rules are nearly identical. (Robots! I hate robots! They're too damn tough! At least in the CRPGs.)

Omegakron, for Lords of Creation by Avalon Hill
Details a post holocaust Akron, Ohio. Nicely done, mutants, mobile org (like Mercury) that recycle materials, gangs and much more.

Legion of Super-heroes vol. 2: World Book, for DCHeroes by MGI A solid solar system and planet generation system, planet backgrounds and campaign background all generic and usable with other games. Science Police and Interstellar Law included. Nice surprise, easy to use and complete results.

Beanstalk, by GDW for Traveller:2300
A beanstalk is the 2300 version of a space elevator. Details about beanstalk construction and 3 scenarios pivoting around their use are contained within. 2300 politics can easily be substituted with XXVc corporate/world politics.

Windriders of the Jagged Cliffs, for Dark Sun, by TSR
Ancient race of halflings practice life energy manipulation to create servant creatures and living tools, ie biotech. Great addition for super science of genie creation. Easily added to existing XXVc campaign with great potential for adding color to Jupiter and/or Saturn systems. Cliffside society would be good add to Venus as well.

Asteroid, by GDW (Traveller compatible)
This small box game details a mission to prevent a mad scientist by destroying earth from his asteroid laboratory. Scrap the rules, keep the characters, maps, geomorphs, scenarios and the story. Very XXVc.

Azanti High Lightening, by GDW (Traveller compatible)
Details a class of heavy cruiser built in the XXVc (and Star Frontiers) style (verticle construction like a skyscrapper). Several scenarios and ship variations. Book and large deck maps usable with miniatures. Excellent!

Gamma Knights, TSR for Gamma World 4th ed.
Add powered armor, personal weapons, ship systems/weapons and robots. Gamma World's version of Battletech, very easy to use as a part of XXVc. Powered armor construction adds weapons and systems just like XXVc does with rocketships. Powered battle armor fits the high tech violence of XXVc campaigns. (A note on powered battle armor - it seems to be some sort of vehicle at one end of the spectrum, and nothing more than high-tech plate armor at the other end. Boba Fett-style armor can easily be simulated with Heavy Battle Armor (with fields or without - it doesn't matter) with a few specialized doodads added on, like my very own wrist rockets. On the other hand, powered armor as in 'meches and suchlike falls more into the province of Rocketjocks, as they're more vehicles than armor per se. Make 'em vehicles if they're large and complicated, and carries a lot of heavy-duty firepower, make a new Dex-based Rocketjock skill called Pilot Powered Armor, and make 'em armor if they're small and Boba Fett-style.)

nearly any Gamma World material, TSR
All editions are at least close in rules, scifi post holocaust Earth fits too.

Practical Planetology, for Spelljammer/AD&D, TSR
New planets and their inhabitants. Normal and strange planets (ring world, dyson sphere, disc world, gas giant, water world). Planets with like environs could add detail to XXVc solar system (ie gas giant). (Remember that "exotic" worlds are unlikely to exist in XXVc per se. For instance, a Dyson sphere is an enormous sphere made out of "exotic" material (because of its enormous strength and such - gravitational stresses and so on), totally encompassing a system's sun. Now, Sol's the only sun in XXVc as XXVc is STL, so Dyson spheres and disc worlds are quite unlikely. Having said that, there are gas giants (Saturn and Jupiter), planets with ring systems (Saturn, Jupiter, Neptune, Uranus), and water worlds (the moons of Europa and Ganymede). The "exotics" in XXVc would be the "bottles", or self-contained orbital habitats that are placed around planets, floating in space, or what have you.)

nearly any AD&D/D&D creatures, PC races, weapons, rules
All editions are rules compatible, monsters become aliens, gennies or mutants. Thief class? Character kits (esp. from Masque of the Red Death)? Psionic and Psionicists. Ranger class? Ninja? Rules expansions and options that apply to scifi settings abound. Spelljammer and Tale of the Comet materials contain directly usable items and rules.

Bughunters, Amazing Engine, TSR
Change campaign backgrounds. Creature and race parallels are apparent. Sinners (synthetic humans) are gennies, role using Dark Sun 5d4 for attributes as sinners are enhanced human clones. Easy to use starship deckplan construction as well, though it is not the "skyscraper" layout described in XXVc.

Metamorphisis Alpha to Omega, Amazing Engine (or original), TSR
Change campaign backgrounds and explore the starship Warren. Original's rules would be closer as they too were D&D based.

nearly any Justifiers, Star Childe, materials
Change campaign backgrounds, beta class humanoids are gennies. Rules are similar but not directly the same. (If I understand this correctly, using Justifiers or Star Childe would mean FTL travel - i.e., Faster-Than-Light travel. As BR is STL (Slower...), FTL travel would mean more systems than just Sol system would be available. Although that might be interesting, BR would then take on a whole new scope, and the fascinating dynamics in power balances (well, I think the whole RAM - Venusians - Lunarians etc. balance is interesting, not to mention very delicate) would disappear to some extent, as there's suddenly not a limited amount of ground to cover (FTL would mean that - theoretically - there's a huge number of systems up for grabs, not just Sol system).)

After the Bomb, Paladium
Post holocaust eastern USA. Rules are fairly compatible. Later volumes detail other global locations.

Teenage Mutant Ninja Turtles and Other Strangeness, Paladium
Mutant creation rules usable for gennie creation. Rules not as compatible as Gamma World 4th ed. Does include rules for intelligent non-humanoid mutants, which would expand possible results. Tech, and equipment usable. (There's a reason why human-intelligent gennies are not provided for in the BR rules. (One exception to that is the Alchemcat, but that's neither here nor there.) And the explanation is simple. Even though BR is a space opera, it tries to keep a certain internal logic in the system. (It's all explained in "No Humans Allowed", in a discussion on gennie creation.) Thus, if you need something to be human-intelligent, you start with a human. Even though the end result might look like a winged starfish, there's a human in there somewhere (probably screaming to get out). If you want hyper- intelligent mice that look like normal mice, BR isn't the place to do it, simply because you need volume to get a human-size brain in there.)

FTL:2448, Tri Tac
Change campaign backgrounds. Faster than light, interstellar trade, espionage, war, exploration and everyday life. Rules are another 1/2 step away from Palladiums as compared to TSR's D&D based games but most cover new ground so using them is easy. Rules are modular adding depth, complexity and subject matter at the GMs discression. Large list of alien races with usable stats. Great background info for militaries, law enforcement, civillian problems, urban decay, high tech splendor, space pirates, etc. (... but it wouldn't be Buck anymore with FTL, or so I think at least.)

Galactos Barrier, Amazing Engine, TSR
Change campaign backgrounds. Interstellar adventures at faster than light speed. Not nearly as expansive, compatible nor with as many options as FTL:2448 but less expensive and more available. Excellent magic/psionic system that's based on music.

Change campaign backgrounds. Rules as compatible as Palladiums with D&D based games. Lost colony world background. (Lost colony world background would again mean some sort of wormhole/jump/warp technology, putting it at (at least previously) FTL level. It just wouldn't be BR, it would be Mach with BR rules. Now that's not necessarily a bad thing, it's just that it wouldn't be BR, IMHO.)

PSI World, Fantasy Games Unlimited
Add psionics to XXVc as well as excellent "psionics in a normal world" background material. Rules are as compatible as Palladiums with D&D based games. Adds another front for RAM and NEO to compete on. Also contains an easy to use body hit location system to add realism without complexity. Lists contemporary weapons with reasonable damage ranges for easy use in XXVc. Maybe genetically altered races (like martians and venusians) cannot be psionic, only pure strain humans can? (Actually, the Oberon Sidhe are the psis of BR, but their abilities might be expanded and differentiated, or one decides to run a "non-canon" campaign. Each to his own, I guess.)

some Star Frontiers and Knight Hawks materials, TSR
Ships are constructed in the same vertical "skyscraper" format. Knight Hawks adds a reasonable possibility for faster than light travel. Miniatures of ships and people are appropriate. Rules are vaugely D&D based and will require conversion work and knowledge of Star Frontiers to do. Knowledge of Knight Hawks rules can easily allow conversion and expansion to XXVc ship combat rules with new weapons and options.

a lot of Alternity, TSR, material
Situation rolls make a good addition, you can even use the free fast play rules. Rules are less D&D based than XXVc or Gamma World but still D&D based all the same. Add to XXVc campaign, or change it.

Villains & Vigilantes, Fantasy Games Unlimited
Super powers, weapons, tech, gadgets and such. Rules as compatible as Palladium with D&D based games. Adventure supplements dealing with space exist and could be used in straight or modified XXVc campaigns. Add superheroes to XXVc or use the powers to detail gennies.

Ships of the Fleet, Volume One
Good news for Buck Rogers: XXVc (Babylon 5 and Star Frontiers too)! I just purchased "A Jovian Chronicles Project, Ships of the Fleet, volume one, JOVIAN CONFEDERATION" (on sale of course, $24 regularly). It contains 3 classes of starship (Destroyer, Carrier (mecha), Tender) with the same gravity assumptions/technology of the above mentioned game(s). Each ship has decks vertically up from the engines, with notes on updates that include spinning habitat area rings. The ships themselves are very reminiscent of B5s Omega class destroyer.

Character sections, maintenance skeds, repair times, emergency procedures and combat tactics are included. As well as, a section on modular ship construction for those more creative. More data than you will ever need is provided along with schematics and deck plans. A sound buy for any SciFi gamer.

Steel Deep: The Luna Sourcebook for MGI's Underground
This is the forth Underground supplement I have had the pleasure to read. That being said, I have yet to see the rules nor understand how such a violent chaotic dystopia of a campaign setting can sustain play for any length of time. Still Luna, like the other supplements I have read, shows quality writing and unlimited plot potential. Luna maintains that dystopia perception with as much "good idea gone bad" as the rest of the Underground universe. At $18 for 124 pages, it is no bargain basement find, but MGI continues to put the highest effort into the quality of these Underground products so that you can feel that they are worth the price. Easier, than ever, to follow text with some very graphic art, complement the layout to perform an excellent job of conveying the detail and the feel of Luna in an Underground campaign. Sidebars provide footnotes for facts that may seem glossed over in the main text.

"Steel Deep" refers to the "honor among thieves" code of conduct at the heart of this Luna colony's transition, from maximum security prison and research facility, to vacation resort and tourist Mecca. According to the campaign timeline, in 1999 (sounds kinda funny now, even though this was published in 1996) the government opened New Solidad as a maximum security prison on the moon. Shortly, it became too costly to run. Corporate interests turned the prison into a research facility, the very facility where extraterrestrial DNA was used to create the first boosteds. As corporate interest and profits dwindled, so did security and maintenance. As a result, the inmates were able to revolt and take control of the prison research colony.

Luna colony is now a tourist center where every indulgence can be satisfied. The cons control their urges and violent tendencies in order to get the tourists dollars. The whole place is ruled by houses, or gangs, of cons. Each house is responsible for some functions within the facility (maintenance, security, etc.) and although they work together, they compete for power and are all out to take over.... eventually. Each gangs colors, hand signs, typical "bangers", habits, rules and responsibilities are provided. Boosteds receive preferential treatment while touring on Luna, in an effort to recruit their services in a gang.

Areas of specific interest on Luna are many. There's an area for settling disputes, where anything goes and the last living participant wins. There is the old lab area, sealed off since the take over, and the monstrous secrets it hides within. There are tunnels, hideouts, casinos, etc. Maps are scarce but the minimum provided is enough to get the less creative GM by.

Rules are included for the new environment, for example, residents wear weighted clothing. Rules for zero and low gravity are detailed, as are the effects of a vacuum or decompression (leak or massive). As always, there are notebook pages in the back, these provide some important NPCs, locals and equipment. All in all, top notch work, as good as all the Underground materials I've seen. Although it is not a necessity, any Underground players would find great use from this product. To a lesser extent than I would like it is useful for other games as well, more useful than most Underground material. The Lunar prison gone tourist center could fit well into almost any future based RPG and easily in such oldies and goodies as TS/SI FREE Lancers, Gamma World, XXVc or PsiWorld.