GPL licensed (22)

AllegroHack

Role-Playing, 2000
Freeware, System component

Buzz Aldrin's Race into Space

Simulation, 1993
Non-playable demo

Catacomb

Also known as Catacombs

Shooter, 1990
Freeware

The Catacomb Abyss

Also known as Catacomb Abyss 3D

First-Person Shooter, 1992
Shareware

Cylindrix

First-Person Shooter, 1996
Playable demo (installed), Full version, Source code

Digger Remastered

Arcade, 1999
Freeware, Extra, Freeware, Source code

Emeritus Pong

Arcade, 2008
Freeware (installed), Freeware, Source code, System component, Freeware

Gladiator

Role-Playing, 1995
Shareware, Source code

Heroes

Arcade, 1998
Freeware

Linley's Dungeon Crawl

Role-Playing, 1997
Freeware (installed), Freeware, Freeware

Mirror Magic

Puzzle, 2001
Freeware, Freeware, Freeware

MULE

Real-Time Strategy, 1999
Freeware (installed), Freeware, Source code

NetHack

Role-Playing, 1987
Freeware, System component, Source code

NetHack Plus

Role-Playing, 1994
Freeware

Open Quartz

First-Person Shooter, 2004
Freeware, Update

Robot Redemption

First-Person Shooter, 2014
Freeware