Pigeon is a 64-bit chess engine, written by Stuart Riffle.
It runs on Windows and Linux, and will plug into any UCI-compatible chess GUI (like this one).
The latest version is rated about 2000 against humans online, at both ICC
and FICS. That's pretty good, you know, for a bird.
When playing against other chess engines, Pigeon has a CCRL 40/4
rating around 1800, which is... somewhat less good. But it always tries very hard.
Full C++ source code is available on GitHub,
under the terms of the MIT license, so knock yourself out.