Faile is an open source chess engine developed by Adrien Regimbald. It is relatively strong, reaching expert or master strength even with moderate hardware. Faile will present a tough challenge and make a good sparring partner for all but the top club players. There are plenty of free chess engines available though, so Faile's main appeal is its source code. Faile is a full featured engine, yet the source is small, clear, neat and well commented.
Faile has been used as a starting point for a number of chess engines, most notably initial versions of Sjeng. It has also been featured in an AI textbook, AI Game Engine Programming by Brian Schwab. Faile has presented me with plenty of fond memories over the years, however I no longer have time for active development on it. That said, I don't want to see it entirely relegated to the dustbin of history either. If you have bug fixes, or suggestions for Faile, please feel free to submit some feedback on Faile's project page, in particular the tracker. Keep in mind that I'd like Faile to maintain true to its focus of being a full featured, yet easy to understand engine.