Free Fruit

One of the best chess programs over the last 5 years has been the program Fruit. Written by Fabien Letouzy, Fruit finished second in the 2005 World Computer Chess Championship, and had a rating of 2842 on the SSDF Ratings list at the end of 2006.
But the most significant thing about Fruit was that up until version 2.1 is was open source. This meant that other developers could take the source code and build there own engines. This some did, resulting in programs like Toga II and Gambit Fruit.
After version 2.1 Letouzy closed the source, mainly due to the worry that other programmers could pass off versions of Fruit as there own in Computer Chess competitions. While version (and below) remained open source and free, later versions could be purchased from the official site.
Now the good news is that all versions of Fruit are freeware. These are versions 2.2, 2.2.1, and 2.3. As Fruit is stronger than Fritz and possibly Shredder, if you are looking for a powerful engine to run under Chessbase etc then Fruit is an excellent choice.
And one more thing. If you are studying Computer Science (or just interested in becoming a better programmer), download the source code for Fruit 2.1 and have a look at it. As a programmer I regard it as a thing of beauty, and anyone wanting to become a brilliant programmer couldn't go wrong in learning from Fabien Letouzy.