...happens to run on Mac OS X and Palm. The Sigma Chess interface is extremely polished, although maybe a bit busy for beginners. A reduced interface for casual players would be welcome, but serious players will discover HIARCS, the chess engine Gary Kasparov considers "better than Deep Blue", and the heavy-hitters will find almost everything they could ask from a computer chess companion in Sigma Chess. The LITE version (!) plays up to about 1925 Elo (the paid and registered full version plays at a mind-bogglingly higher level), and the Lite edition library includes Bobby Fisher's games from the early 70's, along with much else. There is even a European version of a kid's chess pieces, which is fun. (But... no Wizard's Chess pieces?)