Yesterday, Jeroen van den Bos showed me Sphere Online Judge (SPOJ), a site with almost 1200 (!) challenging programming problems / puzzles, and which allows you to upload a solution in sourcecode form and which is compiled and tested on the server. The site exists for over 2 years already and has over 2600 contestants trying to get to the top. That's right, for every puzzle you solve, you get points and the more points you get, the higher your ranking. .
Although the site runs on a Linux box, you can provide solutions in C# (or in a lot of other languages if you want) which is then compiled and ran on Mono. Every problem gets input from stdin and emits output to stdout, so the site can check if you indeed solved the problem. It also times your execution time, and if you fail to provide a valid answer in the problem's time limit (sometimes you just have 1 second, so often brute force is out of the question), your answer isn't correct as well.
What's so great about the site is that it forces you to think harder than you normally would to solve a problem: you really have to be creative and think out of the box, as the easy-way-out solutions aren't going to help.
So if you have nothing better to do during this holiday or if you need some brain massage, or are just looking for some nerd fun, check out Sphere Online Judge (SPOJ). I haven't registered yet, but are planning to do shortly. Happy coding!