You can donate the spare processing power on your computer via one of the dozens of ongoing volunteer computing projects, many based on open source software called BOINC. Just by using a screen saver!
The grid supports research into cures for muscular dystrophy, influenza, AIDS and childhood cancer, among other things. The grid, which was headed for half a million members as of August 2009, runs on BOINC software and is funded by the NSF.
World Community Grid