Crafty 22 is (to the best of my knowledge) the strongest free engine ... it's good.

Toga II technically has a higher rating than Crafty and plays more agressive chess, it is also free, though I have not used it.

You can get Deep Rybka 2.2 and Winboardx complete with installation from Resources/Downloads/Download Free Chess Stuff on this site. Here's the link:

http://www.chess.com/download/view/portable-winboardx--rybka-22---updated-setup

So far as I know this is an old version of Deep Rybka which is available free of charge. The only caveat is that you need a dual core or better processor to get the best results.

http://en.wikipedia.org/wiki/Chess_engine#Freely_available_chess_engines

links to free engines

http://www.superchessengine.com

XBOMB DEFEATED all of the suggested engines here.

 http://chessbombsite.mylivepage.com/forum/1102_General/765_XBOMB_STRONGEST_FREE_SUPER_CHESS_ENGINE

Highest rated of all the free chess engines.

http://tech.groups.yahoo.com/group/Chess_Engines_Testing/message/2107

It even has its own music video...

http://www.youtube.com/watch?v=b6uraNZnD9M

XBOMB DEFEATED all of the suggested engines here.

 http://chessbombsite.mylivepage.com/forum/1102_General/765_XBOMB_STRONGEST_FREE_SUPER_CHESS_ENGINE

Highest rated of all the free chess engines.

http://tech.groups.yahoo.com/group/Chess_Engines_Testing/message/2107

It even has its own music video...

http://www.youtube.com/watch?v=b6uraNZnD9M

Stockfish 1.5.1 is the best free engine and it works on multiple platforms. Try it.

Xbomb destroyed stockfish 151

xbomb  beats stockfish151  2.5-1.5 , xbomb blanked stockfish151 4-0

download the game

http://www.mediafire.com/?sharekey=85d5278f8ea48761c2b435915e8821d7e04e75f6e8ebb871

Xbomb still the best free chess engine

The strongest free chess engine is XBOMB.

http://chessbombsite.mylivepage.com/forum/1102/765_XBOMB_STRONGEST_FREE_SUPER_CHESS_ENGINE

Try it to believe it...