GNU Chess

GNU Chess is a free software chess engine which plays a full game of chess against a human being or other computer program. The goal of GNU Chess is to serve as a basis for research. It has been used in numerous research contexts.
GNU Chess is free software, licensed under the terms of the GNU General Public License version 3 or any later version, and is maintained by collaborating developers. As one of the earliest computer chess programs with full source code available, it is one of the oldest for Unix-based systems and has since been ported to many other platforms.
Features.
As of 2 September 2017 GNU Chess 5.60 is rated at 2813 Elo points (when using one CPU) on CCRL's 40-moves-in-40-minutes list. For comparison, the strongest chess engine in the list using one CPU, Strelka 5.5, has an Elo rating of 3108 (the 295 ELO point difference indicates that Strelka 5.5 would beat GNU Chess 5.60 in about 85% of games). On the same list, Fritz 8 is rated at only 2701, and that program in the 2004 Man vs Machine World Team Championship beat grandmasters Sergey Karjakin, Veselin Topalov and reached a draw with Ruslan Ponomariov.
The IQ6 test suite (a collection of chess problems from Livshits's book "Test Your Chess IQ") indicates that on a single core of an Intel Core 2 Duo CPU GNU Chess performs at the senior master/weak international master strength of 2500+ on the Elo rating system.
It is often used in conjunction with a GUI program such as XBoard or GNOME Chess, where it is included as the default engine. Initial versions of XBoard's Chess Engine Communication Protocol were based on GNU Chess's command-line interface. Version 6 also supports the Universal Chess Interface (UCI). Since version 6.1 GNU chess supports a graphical mode for terminal emulators.
History.
The first version of GNU Chess was written by Stuart Cracraft. Having started in 1984 in collaboration with Richard Stallman prior to his founding of the GNU Project, GNU Chess became one of the first parts of GNU.
GNU Chess has been enhanced and expanded since. Versions from 2 to 4 were written by John Stanback. Version 5 of GNU Chess was based on the Cobalt chess engine written by Chua Kong-Sian.
In 2011, GNU Chess transitioned to version 6, which is based on Fabien Letouzey's Fruit 2.1 chess engine. According to CEGT version 5.60 of this code base is stronger than Fruit 2.3, the latest version of that chess engine.