Stockfish is a free UCI chess engine derived from Glaurung 2.1. It is
not a complete chess program and requires some UCI-compatible GUI
(e.g. XBoard with PolyGlot, eboard, Arena, Sigma Chess, Shredder, Chess
Partner or Fritz) in order to be used comfortably. Read the
documentation for your GUI of choice for information about how to use
Stockfish with it.

Stockfish is free and uses the GPL license. You can download, read, 
and modify the source code.

WWW: http://www.stockfishchess.com/
