Board

3Dc (C, GPL-2.0, mature, inactive since 2000)
British Bingo (JavaScript, GPL-3.0, mature)
Catan Dice Game (Java, GPL-3.0, mature)
Chess3D (JavaScript, Python, GPL-3.0, mature, inactive since 2016)
Crosswords (Java, GPL-2.0, mature)
Deer Portal (C++, zlib, beta)
Domination (Java, GPL-3.0, mature)
DreamChess (C, GPL-3.0, beta)
glParchis (Python, GPL-3.0, mature)
GNU Go (C, GPL-3.0, mature, inactive since 2011)
Hnefatafl (C, ISC, beta, inactive since 2014)
Holtz (C++, GPL-2.0, mature)
Ithaka Board Game (Java, GPL-3.0, mature)
JSettlers (Java, GPL-3.0, mature)
Magic Gardeners Tournament (Python, GPL-2.0, beta, inactive since 2007)
OGS Mahjong (C++, GPL-3.0, mature, inactive since 2016)
Open Legend RPG (None, Custom, mature)
Pachi (C, Python, GPL-2.0, mature)
Pasang Emas (Vala, GPL-3.0, mature)
Pentobi (C++, GPL-3.0, mature)
Pioneers (C, GPL-2.0, mature)
Quatter (C++, GPL-2.0, mature, inactive since 2018)
Scrabble3D (Pascal, GPL-3.0, mature, inactive since 2015)
The Bub's Brothers (Python, MIT, mature, inactive since 2013)
VASSAL (Java, LGPL-2.1, mature)