So.... Looking around there seem to be allot of Distributed Computing efforts. Some of them even pay... But which ones are valid and which ones are just taking advantage of users to add their machines to a botnet for the likes of luzsec to abuse.
Seems the Popular ones are:
SETI
http://setiathome.berkeley.edu/
(Voluntary)
Folding@Home
http://folding.stanford.edu/
(Voluntary)
Bitcoin
http://www.bitcoin.org/
(Potential Payout)