Games that have been identified as having proprietary or unique codebases (the root of a tree, if publicly distributed).
“Mordor is a game using the Mordor Codebase with a Tolkien theme. Homepage, Listing on The Mud...Mordor
“ForgottenMUD is a bilingual French/English MUD based on the Dungeons& Dragons 3.5 rules. It's...ForgottenMUD
“Materia Magica (sometimes referenced as MM), originally named Moongate, is a well-known custom...Materia Magica
“MUD1 (referred to as MUD1, to distinguish from its successor, MUD2) is the oldest virtual world...MUD1