We have a number of members currently contributing to a very positive cause. If you are leaving your systems on, just whiling away, consider adding the Folding @ Home project from Stanford university and help to find cures for diseases with your spare cycles.
CPU's, PS3's and GPU's can all be used but GPU's have the most computational power currently it appears.
The new 2xx series cards are beasts at Folding, even at 50% usage. When playing games, simply pause folding and you are good to go
Give it a thought.