"The first Multi-User Dungeon was usually just called MUD, and was written in 1978 by Roy Trubshaw, a student at Essex University in England, originally in the MACRO-10 language for a DECsystem-10 computer. MUD was the first adventure game to support multiple users. The name was chosen partly as a tribute to the DUNGEN variant of Zork, which Trubshaw had greatly enjoyed playing. Trubshaw converted MUD to BCPL, and then handed over development to Richard Bartle, also a student at Essex University in England (see Early MUD History and Interactive Multi-User Computer Games). The success of that game then spawned a number of similar developments across Britain, including AMP, Gods, and Shades." [source]
I find this interesting because the same idea has become very popular these days in gaming, but now there are fancy graphics. Essentially, the idea is the same, and the level of interactivity probably has increased only marginally, if at all.