Here is a list of chess engines that run under Mac OS X or Darwin. Some of these engines cannot be downloaded as Mac executables and, instead, must be compiled by you. It is possible to run Linux binaries and Windows executables with proper configuration and/or emulation software. The Sigma Chess website maintains a list of downloadable UCI engines which have been tested to work in the Sigma Chess interface. Stockfish also provides a nice Cocoa interface for UCI engines.
* GopherCheck
This list of Mac engines was tested on a Core 2 Duo at blitz time controls up to October 2011.
Commercial engines Shredder, Deep Sjeng, Chessmaster, and Ruffian are untested. Other engines in the list are weaker.
Thanks to Norbert Raimund Leisner for creating this list! Thanks to Kenny Dail for adding more engines. Ian Osgood provided the strength ranking and UCI links.