Ethan Blaze
Here's a list of where to download the best free chess engines. All of them are very strong, so watch out!
Source: http://hubpages.com/games-hobbies/Best-Free-Chess-Engines
Stockfish is a powerful and open source chess engine.
Fire 4.0 chess engine download
GullChess download.
Free version - Houdini 1.5a
Commercial Version - Houdini 4
Rybka is a UCI chess engine programmed by International Master Vasik Rajlich. Rybka is the chess engine being used for analysis by most top grandmasters. Garry Kasparov on Rybka: 'Rybka is a phenomenal improvement. Rybka is really good. Really good.' Grandmaster Larry Kaufman is part of the Rybka team and helped improving Rybka's evaluation algorithms.
You need a uci-compatible chess program to run these chess engines. You can find out more about chess engines and how they interact at this link!
