Library of Effective GO routines. Avaliable at http://www.mimuw.edu.pl/~lew.
A bit text from its README:
This is a Library of Effective GO routines (EGO library / libEGO), a high performance implementation of Go rules and most popular algorithms used in computer Go. This is also fully functional GTP Go engine.
Author: Lukasz Lew (and some help from others)