United we compute!
Probably the most well-known and popular instance of distributed computing is the SETI@home screensaver project. But there are so many other organizations out there with equally(?) noble causes just itching to borrow some of your unused CPU cycles. Math geek? Help look for the next Mersenne prime number. Cryptography buff? Help push the cutting edge of RC5 code-breaking at Distributed.net. Want to conquer the financial world? Help find patterns in the stock market.
Here's a good list to start shopping for your favorite project.