Rybka 2.3.2 / 2.2 / 1.2
Arriving in December 2005, Rybka has proved to be a revolution in chess engines, with the latest commercial versions topping all the rating lists by a large margin. Programmed by an international master, it has a good evaluation function and provides accurate middle-game assessments where other engines often go astray. This free version, released at the same time as Rybka 3 (Aug 2008), is now the highest rated free engine. Following the practice introduced with Rybka 1.0 beta, it was also more highly rated than other commercial offerings! Quite frankly, this is as good an engine as you'll ever need for analysis! For playing against, it has the useful Limit ELO strength parameter, but doesn't have a built-in time lag, so will reply almost instantaneously. Note that the true node count and depth are obscured - add 3 to the displayed depth. Also available for free is the hand-tuned RybkaII.ctg opening book and earlier versions of Rybka, inluding the highly regarded (for endgames) 1.2f and the winfinders.
Now (May 2009), to complete the series, Rybka 2.3.2 (32-bit, single processor only) is included with ChessOK's free interface for its training courses, Peshk@. This version was top of the rating lists for about a year until the release of Rybka 3, and even now is second only to Rybka 3 and Naum 4!! Navigate to the Peshka folder and copy the engine to whereever you keep your engines!
http://www.rybkachess.com/index.php?auswahl=Download+demoToga II 3.1.2 / 1.3.1, Grapefruit and Cyclone
Based on the ground-breaking open source engine Fruit 2.1, this engine adds another 100 Elo points to sit close to the top of the rating lists. At the same strength as the best commercial engines (apart from Rybka and Naum4!) this is an essential download! I find the analysis and evaluation very reliable, and the play attractive. There's also a slightly stronger version, 1.4.1, that doesn't support multipv, and a couple of other equally strong derivatives called Grapefruit and Cyclone. http://computerchessengines.mylivepage.com/
Pro Deo 1.6
The last free version of the formerly commercial program, Rebel. This has a very human-like playing style, which means that it's not only satisfying to play against, but also very good for providing sensible analysis. It comes with some interesting personalities with different styles of play: I particularly like playing against Tal and Q3! Tip: replace the wb2uci.eng file with this one, so that the personalities can be accessed directly from the GUI. There are some additional opening books here. http://www.top-5000.nl/dl.htm?file=dl/ProDeo16.zip
Thinker 5.4
This is easily the most entertaining engine out there at the moment, in the style of Mikhail Tal. If it doesn't sacrifice a piece every so many moves, I think it gets a headache! A unique style, and able to compete with (and confuse) the strongest engines! Set it up in an engine match, sit back, and enjoy! There are frequent updates, some seemingly with more sacrificial pizazz than others. It has been climbing up the rating lists and the latest version is now the strongest free engine after Rybka 2.2. A rather huge downside is that the engine does not display any analysis!! It's a winboard engine (like Pro Deo), so it either needs to be installed as a winboard engine or used with the wb2uci adaptor. On the website, as well as the winboard engine, there is also a very simple GUI version, and a Pocket PC version with a FICS interface.
http://cid-2991af457de54bf0.skydrive.live.com/browse.aspx/PublicGlaurung 2.2 / Stockfish 1.5.1
Glaurung is a strong, open-source engine with an interesting, attacking style (Way too many UCI options, though!). Even stronger is Stockfish, an impressive engine based on Glaurung, that seems to have a particularly agressive search, and is improving by leaps and bounds. It wouldn't be too surprising to see it become the strongest free engine, were it not for R....!
http://www.glaurungchess.com/Homer 2.0.1
Say "Hello" to Homer! This is a good engine for playing against, with adjustable styles and some pre-programmed personalities. It also has the very useful limit strength UCI option. I hope the next version gets to say "DOH!" when you beat it! ;)
http://www.homer-chess.com/Fruit 2.2.1 / 05.11.03 / 2.3
Formerly the strongest chess engine in the world, this is now one of the strongest free engines, and makes a good no-nonsense analysis partner. Many, if not all, the commercial programs benefitted from the open-source Fruit - here's your chance to see how the original programmer developed it!
http://www.fruitchess.com/Delfi 5.4
After Pro Deo, this engine has one of the more human-like playing styles, which can be good to play against. It is possible to limit the strength to approximately a 1000 ELO player in the UCI options, which is good for beginners. Also available is Delfi GUI and a Pocket PC version.
http://www.msbsoftware.it/delfi/Other...
Strong engines worth downloading include Spike (which works as both a UCI and a winboard engine), Naum, Alaric, and Bright. Slow Chess Blitz and Queen are engines that display their own interface when not used in another GUI.