1: TopCoder. The world largest programming compete website. It hosts SRM for contest.
2: CodeChef. It is hosted by an Indian Company, claimed to be non-profit.
3: SPOJ. It is an online Judge system which support more than 40 programming languages.
4: CodingBat. It only support Java and Python.
5: Google Code Jam: It is hosted by Google. Some Googlers are asked for participating for test before its being publicly accessed.
6: Dream In Code: It is more like a forum.
7: Codeforces: Online Judge but allows competing with other register users.
8: UVa Online Judge. Hosted by Valladolid University. It archives almost all the ACM competition problems
9: Python Challenge: as the name tells.
10: Facebook Puzzles: it is hosted by Facebook.
11: ACM-ICPC: the world largest ACM competition.
Others: LeetCode, Hackers.org.