Pigeon is an open source chess engine, written in C++ by Stuart Riffle.
It runs on 64-bit Windows and Linux, and will plug into any UCI-compatible chess GUI. A lot of the core code is branch-free, which allows it to process multiple positions in SIMD.
The current version (Pigeon 1.5.1) 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 of about 1750, which is... somewhat less good. But it always tries very hard.
Pigeon is released under the MIT License - see LICENSE.txt for details.