AtCoder is a programming contest site for anyone from beginners to experts. We hold weekly programming contests online.

CodeChef is a competitive programming community of programmers from across the globe.

Google's Coding Competitions (Code Jam, Hash Code, and Kick Start) enthrall, challenge, and test coders around the world. Try your hand at one, or all three.

Codeforces is a website that hosts competitive programming contests.