Computer programs which play go.
By Rémi Coulom, distributed commercially by Unbalanced Corporation.
Platforms: Windows, iOS, Android.
A strong open source go engine, initiated and mainly developed by Petr Baudis
Drago is an SGF viewer/editor which you can also use to work through problems (like uliGo) and to play against GnuGo. Windows. Freeware.
An open-source minimalistic Go Engine, based on Monte Carlo playouts, written in Python.
The algorithm is between 500 and 600 lines of code only. This is a great source for understanding how to apply Monte Carlo type algorithms to building a go playing program.
By Petr Baudis, the creator of Pachi.
A strong Go playing program combining advances in Go programming and further original research into a small, easy to use graphical interface.
Freely available. Graphical interface for Windows. Commandline engine with GTP interface for Windows/Mac OS X/Linux.
Currently the strongest Go Engine, by DeepMind - not available for playing, but they have some of the game records of AlphaGo's games
Links to many go playing programs
Written by Anders Kierulf. Commercial (there is a free demo version). Windows.
A free, open-source, go-playing program.
Commercial go playing engine. Windows.
Seems a little outdated now (2016)
Written by David Fotland. "Many Faces" was one of the first go programs. Commercial (but see the free 9x9 version, Igowin). Windows.
A free 9x9 version of David Fotland's program "The Many Faces of Go". Windows.