Another very strong chess engine is Houdini. The current version is widely considered to be the strongest chess engine available. As of December 17, 2010, Houdini 1.5 was ranked #1 on the leading IPON computer chess rating list, 50 Elo ahead of Rybka 4 and 100 Elo ahead of Stockfish 1.9 and Critter 0.9.
Houdini is free for non-commercial use. It can be downloaded from the Houdini Chess Engine website.
There you can also find more information about the name, usage and background of this strong chess engine.