Distributed computing: Most of the time your PC is on it will be only using part of it's potency. The rest goes to waste. You can help the search for aliens, predict climate change, fold proteins to perhaps help our undertstanding of how these might form cancers or other diseases, search for pulsars in space, and all sorts.
Look at the rather messy sticky
http://www.cableforum.co.uk/board/sh...ad.php?t=13051 for more information.