Project Euler
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.
Code Golf
Code Golf allows you to show off your code-fu by trying to solve coding problems using the least number of keystrokes. Challenges are always open, and your entries are automatically scored so you can start playing right away!