This is from a post of mine from last year. These were my top 10 free chess engine recommendations at the time. (as of 12/7/13)
Stockfish DD: http://stockfishchess.org/
Komodo 5: http://komodochess.com/
Houdini 1.5a: http://www.cruxis.com/chess/houdini.htm
Critter 1.6a: http://www.vlasak.biz/critter/
Gull 2.3: http://sourceforge.net/projects/gullchess/
Bouquet 1.8: https://sites.google.com/site/chessbouquet/
Rybka 2.3.2a: http://www.rybkachess.com/
Strelka 5.5: http://en.wikipedia.org/wiki/Strelka_%28chess_engine%29
Protector 1.5: http://sourceforge.net/projects/protector/
Crafty 23.8: http://www.craftychess.com/
If you wish to purchase an engine, you probably can't go wrong with the latest versions of Houdini and Komodo.
A list of the current strongest engines can be found here.
Chess engines can be beneficial to players who aspire to become stronger. They provide analysis, recommend moves, show the most popular opening lines, etc. I have been thinking about getting an engine that is stronger than the one that I currently have (its maximum rating strength is approximately 2500.) Which chess engine is recommended for improving players? Also, which ones offer the most for their cost?