Cengix (formerly known as yacen)
https://code.google.com/p/cengix/

Cengix (chess engine no. x) is a simple chess engine that uses the UCI interface to interact with any compatible chess GUI. Its main focus is understandability of code and techniques rather that pure performance. cengix is written entirely in ANSI-C using nothing more than just the standard library and thus should run on almost any platform that has a C compiler available.