Arcade

Battle Tanks (C++, GPL-2.0, mature, inactive since 2009)
Blobby Volley 2 (C++, GPL-2.0, mature, inactive since 2017)
Bombermaaan (C++, GPL-3.0, mature, inactive since 2010)
Fluid Table Tennis (JavaScript, MIT, mature, inactive since 2013)
GPL Arcade Volleyball (C++, GPL-2.0, beta, inactive since 2006)
Maelstrom (C, C++, GPL-2.0, mature, inactive since 2002)
OpenLieroX (C++, LGPL-2.0, mature)
PainTown (C++, Java, Python, 3-clause BSD, mature)
Pang Zero (Perl, GPL-2.0, beta, inactive since 2007)
SDL Asylum (C, GPL-3.0, mature, inactive since 2009)
Tux Football (C++, GPL-2.0, beta, inactive since 2012)