Glaurung and Scatha are two simple open source chess playing engines. The two programs are almost identical, except for the fact that Glaurung plays normal chess, while Scatha plays Wladyslaw Glinski's hexagonal chess.
Glaurung is available as a free UCI (the "Universal Chess Interface") chess engine for Mac OS X, Linux and Windows. You will need an UCI compatible GUI in order to use it. Alexander Schmidt's page is a good place to look for information about UCI compatible chess engines and GUIs.
For Mac OS X, Glaurung and Scatha are available with a GUI. You need a PowerPC Mac running OS X 10.3.9 or a newer version in order to use it. The GUI supports normal chess, FRC (aka Chess960) and hexagonal chess, and is compatible with the UCI protocol. This means that it is also possible to install other UCI chess engines for OS X (e.g. Fruit, Hiarcs, Homer, Ruffian, Shredder or Sjeng) and use them in the GUI. The GUI does not work on the new Intel Macs, and it is not likely that this will change in the near future.
Some full size screen shots are available on a separate page.