Houdini 1.03a
An Ippolit derivative (see below), but with some real improvements. The latest version supports multi PV, and has overtaken Rybka 4 on the IPON rating list. This is a fantastic engine, and is my top choice for analysis.
http://www.cruxis.com/chess/houdini.htmStockfish 1.8
Stockfish is an open-source engine based upon the aggressive Glaurung. It has been the most impressively improving engine of 2009 and is now one of the top few engines in terms of strength. The latest version is reported to be a touch stronger than Rybka 3. A stunning achievement for open-source and an essential download!
http://www.stockfishchess.com/Rybka 2.3.2
Programmed by international master, Vasik Rajlich, Rybka has proved to be a revolution in chess engines, with the latest commercial versions topping the rating lists, often by a very large margin. This version was top of the rating lists from June 2007 until the release of Rybka 3 in August 2008. Even now (June 2010) it has the edge on all the other commercial engines except Naum! Also available for free is the hand-tuned RybkaII.ctg opening book. This is as good an engine for analysis as you're ever likely to need!
http://www.rybkachess.com/index.php?auswahl=Demo+versionIvanhoe / Fire / Robbolito
At the end of 2009, Robbolito and other Ippolit derivatives currently seem to be the strongest engines available. There has been a lot of contention regarding its origins, with some claiming that it is based upon a reverse-engineered Rybka; however, there has been little in the way of compelling evidence, and Vasik Rajlich has remained silent on the issue. I've included it here because its source-code has now been available for some time, and is likely to be used by other engine authors. n.b. these engines aren't included on the CCRL and CEGT rating lists due to the alleged cloning. There's an interesting discussion on the subject here. We'll have to wait to see if they show any improvements...
Update July 2010: Vasik stated that "Ippolit is disassembled Rybka 3 with changes" in response to an email from Sven Schule, published on CCC. On the other hand, "BB" produced evidence that the differences between Ippolit and Rybka are substantive, as discussed on OpenChess. http://www.chesslogik.com/index.htm
Komodo 1.2
Another strong newcomer in 2009, Komodo (formerly known as Doch) is being developed by Don Dailey with the help of Larry Kaufman, who did much of the evaluation work for Rybka 3. Watch this space, as he hopes it will make the top 3 engines soon!
http://komodochess.com/Deep Junior 11.2 demo
It's great to see a useable free version of this top commercial engine! Famous for its sacrificial style and knowledge of compensation, Junior often gives noticeably different analysis from other engines, making it a useful source for ideas. The free unactivated version is limited to 16 ply, which is enough to be useful in analysis, and more than enough for playing against! (If you have trouble with multiPV using 11.2, try 11.1.)
http://www.hiarcs.com/pc-chess-junior-download.htmToga II 2.0 / 3.1.2 / 1.3.1
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 a similar strength to the best commercial engines, this is an essential download! I find the analysis and evaluation very reliable, and the play attractive. http://www.chesslogik.com/toga.htm
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.rebel13.nl
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://chessthinker.tripod.comHomer 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/Glass 1.6
An engine with a full range of personalities for playing against.
http://www.pkoziol.cal24.pl/glass/Hiarcs 13.1 demo
Another commercial engine with a fine pedigree is Hiarcs. The demo is a slowed-down version with multipv and limit elo disabled. So it's not much use for analysis, but may be worth a game.
http://www.hiarcs.com/pc_download.htmOther...
Strong engines worth downloading include GullChess, Critter, Spike (which works as both a UCI and a winboard engine), Naum, Alaric, Bright, and Spark. Slow Chess Blitz and Queen are engines that display their own interface when not used in another GUI.