The following games are free to play.
Colossal Cave Adventure (1977): the first interactive fiction game
MUD (1978): also called MUD1, is the first Multi-User Dungeon
MUD2 (1985): the sequel to MUD
Avalon (1989): one of the oldest running MUDs
Genesis LPMud (1989): famous for its free codebase
DikuMUD (1991): famous for its free codebase
Legends of Terris (1994): a MUD with some graphical features
Meridian 59 (1995): the first 3D MMORPG
Materia Magica (1996): a long running, original MUD
Nightmist (2000): the longest surviving graphical MUD
Ember Online (2017): the most popular graphical MUD
AI Dungeon (2019): the first MUD ran by a neural network
Influences: a list of all the influences on the SRD
Appendix N: a list of literary influences on the SRD
This links to the game mechanics used in Dungeon Seeder.