Houdini 1.5 is excellent and free. You also need a GUI (graphic user interface). I've heard Stockfish is a great engine too. 2 popular GUI's are Arena and Scid v. PC. All can be downloaded from chess.com right here:
http://www.chess.com/downloads
I use an older version of Fritz (cheap) loaded up with Houdin 1.5a and HIARCS ($50) so I can't say anything from personal experience about what's the best free stuff but try a "search forums" and you'll find lots of previous threads. Have fun!
okay thanks!
Try Stockfish DD:
It's stronger then Houdini 3 and almost equall to Houdini 4.
Yes Stockfish DD can also utilize eight cores (4 physical 4 virtual). Beware since your mouse cursor will slow down if on at all cores so maybe 7 is best. I have a Haswell i7 so it's the best current and reasonably priced processor. It still doesn't find Staunton's objectively best Nh1!! unless I scroll past the move however.
Hi everyone,
I need a chess engine I can run to evaluate my games and improve my gameplay. I can't afford Houdini so what other options are there that are free with a high analysis strength?
Stockfish 5 is way better than Stockfish DD. Strongest engine of 2013.
Oh and for your GUI I suggest that you get Lucas chess, but it's buggy. Winboard's way too confusing.
http://www-lucaschess.rhcloud.com/
okay thanks!
spikestars: Beautiful.
So many posters at chess.com won't take yes for an answer.
dude, you don't need to be rich to have a chess engine 
Hi gis? Fancy a Challenge? Think you can top the SFTC rankings? Come and give the Stockfish Time Challenge a go! Go here for more deails http://thestockfishtimech.wix.com/sftc
Hi everyone,
I need a chess engine I can run to evaluate my games and improve my gameplay. I can't afford Houdini so what other options are there that are free with a high analysis strength?