Project Euler

Project Euler is a lot of fun. They post challenging mathematical/computer programming problems for users to solve. Some are quite easy, some are very challenging. I go there to challenge myself. I first solve the problem, usually in J, Python, Ocaml, R, Oz, or Euler (I do not use C++, I write C++ programs professionally, for fun I work with something else). Then I usually try to find a different way of solving it in another language, then I go to the forum (which is only open to users who have solved the problem) and see how others have solved it. Frequently my first attempt does not make the one minute requirement, but I refine it until it does. Maybe I'm weird, but I think it's fun.