The Chess Variant Pages Best of the Net
Huge site with high-quality information on chess variants. Covers ancestors of modern chess.

Zillions of Games
'The first infinitely expandable PC gaming system.'

Chess in the Round
Two player game on a round surface that looks like a dartboard. Twice as many pawns as usual. From White Bear Enterprises.

Chessapeak
A four player game on a square board using the traditional rules.

Three Man Chess in the Round
Three players on a round board use the traditional rules.