This post was published 5 years 7 months 22 days ago. Therefore, it may well be out of date. Do not reply on the contents of this post being accurate. 
World Community Grid now has a Mac OS X version available (PPC only though by the look of things).
I’m not sure how long it’s been available as I haven’t checked for a couple of months or so.
WCG Download page – Windows, Linux and now Mac OS X too