Ralph Churchill is developing Chuchomud. He's using the network parts of TeensyMud.
Reyn Vlietstra is developing ruby-mud home or ruby-mud sourceforge.
Michael Granger and others are working on MUES which stands for Multi-user Environmental Server.
Michael Manlief and others are writing Rbmud.
Travis Smith is working on Muddy Gems MUD Driver.
Bob Aman is planning Mudclay.
James Costlow is developing Multi-User Unified Network MUUN.