Thirty-two years ago, David Horne wrote a chess program [1] for the ZX-81 [2]. It didn't play a great game of chess, and you can't castle, capture en passant nor promote a pawn, but it did have one redeeming feature that set it above every other chess program—it took less than 1K (kilobyte) of space [3]! The program, in its entirety, is only 672 bytes in size.
But there's a new contender for the smallest chess game (and the same limitations—no castling, no en passant, no promotion) with BootChess [4], which is an incredible 487 bytes in size [5]!
[1] http://archive.org/stream/your-computer-magazine-1983-02/YourComputer_1983_02#page/n99/mode/2up
[2] http://en.wikipedia.org/wiki/ZX81
[3] http://en.wikipedia.org/wiki/1K_ZX_Chess
[4] http://www.pouet.net/prod.php?which=64962