ChessWorks: Multiplayer chess game that is playable over the internet or a LAN. [Macintosh/Windows]
Full ZX-81 Chess in 1K: Describes a 672 bytes program that plays chess.
Game Industry News - Learn to Play Chess with Fritz and Chesster: Reviewed by Karen Rosenberry, score: 5 out of 5. Includes screen shots.
Ghost: Ghost by Philipp Claßen. Ghost is an amateur chess program written in C++. Runs on Windows and Linux.
Kuro5hin: Assembly program by David Horne, which is run on the ZX81 with just 1k of memory.