Google Code Jam

• annual competition

ACM ICPC

• annual competition

ICF PC

• annual competition

USA Computing Olympiad

• annual competition
• community
• training

Facebook Puzzles

• competition

SPOJ

• competition

Hack-Fit

• competition
• hackathon

Kaggle

• predictive modeling competitions
• community

Hacker Rank

• competition
• community
• training

Code Chef

• competition
• community
• training

Code Forces

• competition
• community
• training

Project Euler

• tests

Initial Round

• community
• training

Online Judge

• training

Know of another one? Let me know