CodeForces

CodeForces is one of the best sites for competitive programming.

Codeforces is a Russian website dedicated to competitive programming.It was created and is maintained by a group of competitive programmers from Saratov State University led by Mikhail Mirzayanov.

Codeforces provides the following main services to all users:

participation in the short (2-hours) contests, so-called "Codeforces Rounds", held about once a week;

participation in educational contests (1.5-2.5 hours), held 2-4 times per month

challenge/hack other contestants solutions;

ability to solve problems from previous contests for training purposes;

"polygon" for creating and testing problems;

kind of social-networking through the use of internal public blogs.