Compute Against Cancer
http://www.parabon.com
Explains in great detail how you can help work out cancer calculations.

Fight Aids at Home
http://www.fightaidsathome.org/
Download a programme to help with Aids research.

Folding Proteins Experiment
http://www.stanford.edu/group/pandegroup/folding/
Help by using your PC to solve the protein folding conundrum.

Numbers and Codes
http://www.distributed.net
Use your computer to help crack cyphers and mathematical problems.