Jon Roberts says:
I thought those who read TuxRadar might be interested in Project Euler. It's an old project, but a cool one for people like me - that is, those who dabble in programming and maths and are looking for a set of challenges to help them develop further.
From the website:
"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."
They have an active forum where you can go for help and advice, but be warned, people posting spoilers will be reminded not to do so as it's against the spirit of the project.
If any one is interested: I'm on problem 3. Not very far, but I've already learned a lot about prime numbers and how to generate them! Now to find a (much) faster solution.