Look at what my computer is up to! It's taking part in a distributed computing project. Details can be found here: United Devices, Inc.™ Projects: About The Anthrax Research Project.
It's a great way to spend your spare CPU cycles. Most of the time, I'm not sitting in front of my computer at home, so it's not really doing anything useful, but now it's doing something good all on it's own. The software used here also alternates to the cancer research project which has been going on for a while now.
I've taken part in the SETI and PRIMEnet distributed computing projects in the past, but I switched to this when I found out about it. These life science projects just seem like a much more worthy cause. Donchathink?