I'm still back in the Houdini 1.5a era myself, but the forums seem to love Stockfish.
Best Open Source (or Free) Chess Engine
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"Stockfish is open source (GPL license). That means you can read the code, modify it, contribute back, and even use it in your own projects."
Stockfish is also consistently either #1 or #2 on the engine rating list. No reason to use anything else. And yes, it works in Fritz, as any UCI chess engine will.
I have a friend's computer science students creating software for me to analyze games as part of a class, and I'd like to know what the best open source engine (or free version of an engine) out there. I'd like the software to be created for Fritz (I don't believe there is a free version), because that's what I use, but there are two problems: 1) I'm unaware of copyright infringement issues associated with altering or using bits of code; and 2) the students prefer not to have to buy the engine, even if there is no issue with #1. As with any engine I have encountered, it needs to give a quantitative assessement of positions.
Any suggestions?