Sigma Chess Macintosh chess program for both OS X and Classic Mac OS
www.sigmachess.com
CHEOPS Chess Opponent Simulator Linux program capable of human-vs-human, human-vs-computer, and computer-vs-computer play.
www.nothingisreal.com/cheops
ARK ANGLES PC Software by ARK ANGLES and other authors from around Australia.
www.arkangles.com
Schroder BV Rebel chess by Ed Schroder
www.rebel.nl