GNU Chess

GNU Chess is a computer program for playing chess, and is thus a computer chess program.

It is free software (in both the no-cost and as-in-freedom senses) licensed under the terms of the General Public License, and is maintained by collaborating developers. It is often used in conjunction with a GUI program such as XBoard.

In 1998-1999 GNU Chess underwent a transition to version 5. Version 5 was essentially a rewrite of GNU Chess to eliminate spaghetti code and replace antiquated data structures with more advanced computer chess implementation techniques.

http://www.gnu.org/software/chess/