A strong chess engine, among the strongest in the world
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, ChessPartner 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.