I’ve been working through some of the problems on this site recently, and thought it would be of interest to some people here: http://projecteuler.net
What is Project Euler? Basically, it’s a website filled with math problems that are designed to be (most feasibly) solved with a computer. Prime number problems seem to be popular, as well as large calculations (number exceeding long data types or long long, depending on the language).
The problems on Project Euler have developed my programming skill considerably, especially in the area of program efficiency.