Ever wanted to contribute to something great? How does helping a group of scientist solve genetic diseases sound? All you need to do is visit this site - Folding@Home; download its client software and run it. After assigning yourself a unique ID (to track your contribution), and an optional team ID (teams compete for ranking), the client software will start to download work packets to process. Its essentially a low-priority program which runs only when your CPU is idle.

Think about it - most of the time your CPU is idle - web surfing, file downloading, writing a document, reading emails, etc. - are all very common activities yet they use little CPU processing time (especially with fast processors). Why not donate those idle time then? If and when you need 100% CPU time to be free (say, gaming :)), the client is easy to shutdown. Just remember to restart it again.