OpenMacGrid is a computing grid built up entirely of Macs, and open to Mac-using scientists everywhere. OpenMacGrid is based on Apple's Xgrid technology, which comes pre-installed with every copy of Mac OS X 10.4 (and later). OpenMacGrid brings the spare cycles of Mac users together with the computational requirements of Mac researchers.
What makes OpenMacGrid different from other distributed computing projects is that it's open. SETI@home, Folding@home and Einstein@home are closed networks. All run a single application and access is restricted.
OpenMacGrid is different. Like the other grids, Mac users can donate their idle time to challenging scientific computations, but unlike the other networks, researchers are able to access this resource with their own scientific applications. The programs running on OpenMacGrid vary from day-to-day, reflecting the projects being undertaken by Mac researchers at that point in time. Mac users directly aid scientists in achieving important scientific breakthroughs.
UPDATE: Another cool feature of OMG (in addition to its acronym) is that you don't have to install any new software to participate. You join by enabling the Xgrid checkbox in the Sharing pane of System Preferences. You can find the details here.
Sounds like a good cause to me.