CodeForces

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.