chess analysis engines

i tried search and did not find anything useful and pardon me for a maybe obvious question but could you folks recommend me some good and free chess engines one could use for analysis?

thx in advance...

Rybka 2.2 is the best free engine, and here it is, in package with WinboardX GUI:

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

many thanks for the answers... i was hoping to find rybka but i thought it wasnt free?

hvala gardelin(:

Empirical data from many engine matches suggest that Rybka 2.2 is by far the best free engine, and even more - it's stronger than most other commercial engines!

http://www.computerchess.org.uk/ccrl/4040/rating_list_all.html

Rybka 2.2 isn't the best; fire and houdini are both rated around 3400 ELO, and are free to download

I recommend Stockfish: http://www.stockfishchess.com/ - it has ratings which seem to put it between Rybka 3 and Rybka 4, plus Stockfish is free and available for Windows, Mac and Linux.  You can use it with your own interface.

Of course Rybka 2.2 is not the best free engine any more, but at the time I wrote that it was! Now (02.01.2011.) Houdini 1.5 is absolutely the best engine available and Stockfish 2.0 is pretty strong too.

http://www.chess.com/download/view/portable-winboard-45b-houdini-15stockfish-20

Slazem se sa njima. Ja sam skinuo Rybka 2.2 sa ovog sajta ides u sekciju RESOURCES tamo mozes da je nadjes, odlicna je za analizu tvojih meceva.

Is there any engine that does automatic annotation of game files? For example, by adding ?? on blunders or !! on mate less than 'x' moves? I would like to save some of these basic analysis on my logbook so that I can study the games while away from a computer. 

Ok, it seems that Chessmaster analyis does that (kind of), altough the output seems to be a little big a repetitive...

Any chess engine which have option of anylasing or evaluating positions is more than good enough for 95% of players here..at least. 

I created one for my own use which uses crafty (free open source, pretty strong chess engine) for analysis. Posted on chess.com about it http://www.chess.com/forum/view/game-analysis/simple-tool-for-game-analysis. You can download it from http://code.google.com/p/java-board-games/downloads/list  A basic user guide is also available at http://code.google.com/p/java-board-games/wiki/UserGuide.

Stockfish analysis chess engine is free online at chesseng.heroku.com