I've just done something new, which I'm disproportionately excited about. You'd think that volunteering to let someone else use my computer would be a problem. However, I've just signed up for Rosetta@home, and what I'm letting people use my computer for is to run complicated algorithms to try to determine how proteins fold and what is the lowest energy configuration for a given protein. These solutions are used by researchers to try to figure out ways to combat diseases like malaria, HIV, and Alzheimer's.
It absolutely charms me that the software you download, to run these problems, is called BOINC (that's Berkeley Open Infrastructure for Network Computing). BOINC runs in the background using CPU when you aren't using it. It's absolutely zero maintenance: it goes out to the Rosetta server, downloads as many problems as it thinks my computer can handle (seems to be two), chews through them, and then when it's done, uploads the results to the server. I have a Rosetta account and get credit for problems solved - think karma, it's not money.
So far I haven't noticed much effect on web browsing and email, although it takes a little longer to open files. Plus, it acts as a screen saver showing you how it thinks the current protein it's working on might fold. Rosetta always needs more computing power - this is an active research facility. So if any of you would like to let the CPU you probably don't use much contribute to serious medical research, let me encourage you to check the site out and sign up. They have BOINC clients for Windows, Mac, Linux, and Solaris. Also, there are other projects you can sign up for once you have the client - SETI@home for instance.