Project Euler is a fun way to kill an amazing amount of time, assuming that you're a computer programmer and enjoy challenging programming challenges.
As of now, there are 182 problems posted, with a new one posted every few weeks. Each problem is designed to be able to run in under a minute on average hardware, assuming you have a clever solution. One example of this is Problem 67. If you just do an exhaustive search of all the possible paths, it will take you longer than the age of the Earth, but with a smart algorithm I was able to solve it in three seconds.
Once you solve the problem, you gain access to a forum for each problem to trade tricks with everyone else who has solved the problem.
I've already solved problems 1 2 7 9 14 15 18 67 92
All of my solutions are in C, but some people solve them just using pencil and paper. Let me know in the comments if you've solved any PE problems.
Edit: I've started posting my solutions to the problems here.