Project Euler: I am 10% geniusProject 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 solved 15 out of the (currently) 144 available problems, so I'm "10% genius". I've been using C++, with the help of my homebrew bignum class. It's a little ugly. I may redo my solutions in Python. Or maybe Ruby. Heck, maybe even Haskell.
Working through the puzzles seems like a good way to learn a new language. Additionally, I've been asked to solve variants of several of these problems on job interviews in the past, so this definitely keeps your mind sharp for that, too.
If you decide to participate, good luck, and have fun!