From their description:

Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.

I’ve done a few of the first problems, implemented them all in ruby so far (or just did the math required), and they’ve definitely been interesting and fun to work on.  4 down, 242 to go.  All of the problems are math related, and you are basically presented with the problem and a field to input your answer.  After you supply the correct answer you’re given a link to the forum (most of which are locked now) where people have discussed their approaches to solving the problem.

If you’re looking to brush up on some math skills, or just need some interesting problems to throw some code at, this site provides some great things to work on.