Project Euler

I came across the Project Euler site the other day. If you enjoy solving mathematical/programming problems then I recommend this site. You're given a number of problems to solve with each problem in the series getting progressively more difficult. If you sign up, you can submit your answer and if your answer is correct then you'll get access to the message boards for the problem you worked on. The problem message boards are where you can share your solution and see solutions by other people who may have used a different programming language. It's interesting to see the C# solutions compared to the solutions in other languages. Worth a look if you have time!