BOINC exists for the Mac.
The following BOINC-projects have Mac-clients: Seti@home, Rosetta@home, Einstein@home, Leiden classical, Rectilinear Numbers Crossing, SIMAP, SZTAKI Desktop Grid and World Community Grid (with five different projects). Thus there are at least 14 different projects to choose from.
Please find more info about the projects (and all the links) in the "Comprehensive BOINC Projects" Thread!
