I am a regular in Achaea, Imperian and now Lusternia (which is in open beta). All three are owned by Iron Realms Entertainment and have some things fairly similar - one of those is that without a doubt its the single best MUD engine I have ever seen, and considering that one of their games can expect between 2-600 people on at any time (on average) it has to be.
Achaea Imperian Lusternia