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.
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.
How it works
Pigeon is released under the MIT License - see LICENSE.txt for details.