MUSE Ltd. - Purveyors of online games since 1985; products are MUD2 and the MUDDLE programming language and development system, both available for commercial licensing.
Open Site - An article with an explanation of what MUDs are, an overview of their history, and a list of variants.
Richard A. Bartle: Players Who Suit MUDs - Discusses whether MUDs are games, pastimes, sports or entertainments and suggests four kinds of player: achiever, explorer, socializer or killer.