- The Underworld [[Silly] Highly customized] (February 13, 2000)
- The Unofficial Squaresoft MUD [[LP] Lima 1.0a6 / MudOS v22.2b12, slight server mods] (July 2, 2009)
- UNItopia [[LP] LDMud-3.2.9 UNIlib-mudlib] (October 1, 2001)
- Ugas Daos [[Custom] Web-Based] (March 27, 2001)
- UglyMug [[Unknown] Heavily customised Tiny] (January 10, 2002)
- Ultimate Legends MUX [[MUX] ] (August 4, 2011)
- Unbridled Desires [[MUCK]] (January 6, 2007)
- Uncharted Peak [[Custom] ] (July 19, 2012)
- UnderDark - Race Wars [[Dikumud] 99% rewritten diku, influences Basternae, Duris] (June 9, 2012)
- Underworld Dreams [[Envy] not stock] (November 1, 2005)
- Unicorn Valley [[Smaug] modified] (December 3, 2011)
- Universe Unlimited [[MUX] TinyMUX] (April 24, 2012)
- Unknown Epicentres [[MUCK]] (March 20, 2002)
- Unwritten Legends [[Custom]] (January 31, 2004)
- Utopia [AberMUD] (June 17, 1998)
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