Sigma Chess is a master strength Macintosh chess program for both OS X and Classic Mac OS.

Sigma Chess 6.1 Lite, which is freeware and can be downloaded here. Certain high-end features are either disabled or limited in this free version. Nevertheless, it's a complete, full strength chess program with many features; it's not just a demo. If you're a casual chess player without the need for special features, chances are that Sigma Chess 6.1 Lite will do just fine.

For only $20 you can upgrade Sigma Chess 6.1 Lite to Sigma Chess 6.1 Pro, which contains the full feature set.