Pachi
Pachi is an open source go playing program or go-bot. It is being actively developed.
See latest go software updates for the latest update news.
Homepage is at http://pachi.or.cz/.
Online webapp http://www.peepo.com with maps and comments as you play
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The source code, written in C, is available at http://repo.or.cz/w/pachi.git.
Windows binaries (include Makefile for MinGW) are downloadable at http://pachi.or.cz/.
The successive versions of the program are named after famous Japanese players:
Pachi 2.00 September 2008 Sana? Pachi 3.00 October 2009 Sanetsu Pachi 4.00 October 2009 Doetsu Pachi 5.00 Februari 2010 Dosaku .... Pachi 8.00 May 2011 Shuhaku Pachi 9.00 October 2011 Hakugen