Stockfish is an open-source UCI chess engine based on Glaurung.

Stockfish is an open-source UCI chess engine based on Glaurung. Stockfish is an alpha-beta searcher and uses a bitboard representation.

Rated at over 3200 ELO on most chess rating lists, Stockfish is right up there with commerical engines like Rybka and Naum.
Cross-platform

Stockfish is available for Windows, Mac OS X and Linux. Want to do chess analysis on the go? We've got an iPhone/iPad app, too.
Free and Open Source

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