Project Euler

Project Euler
http://projecteuler.net/

Project Euler is a set of mathematical or computer science-related problems. It doesn't matter how you get the answer, as the inputs are given. Users are ranked not purely by how many problems they solved, but each problem is given a score - how much it's worth - depending how many users solved it.

The site is an upgrade of an older site which has a different ranking system and website: http://mathschallenge.net/index.php?section=project.