Programming contest sites
This page is probably outdated.
Programming contests:
Topcoder
Valladolid contest hosting
USACO contest gateway
Programmer of the month
Al Zimmermann's programming contests
Links to other programming contests
Bitwise
IOPC contest
Lars' programming contest
Internet Problem Solving Contest
ICFP programming contest

 
Robot judges:
Valladolid online judge
Sphere online judge
Ural State University online judge
Peking University online judge
Saratov State University online contester
Zhejiang University online judge
Programming challenges

 
Other problem solving sites:
Mathschallenge.net (Project Euler)
Geek challenges

 
Training resources:
USACO training program gateway