Altarus

My favorite MUD is called Amitoune - Way of the Amibitious. The official features list is given below, but the MUD offers far more than what is given. For instance, you can hire mobs to guard your house and build a jail for trespassers. You can choose a mother and father race, and inherit racial skills. Another cool feature is pets; you can hire a pet and it levels, gains abilities, and evolves.A much more extensive features list (over 5 pages) can be found on their website, but here is the "short" list:

Housing - player owned and customizable housing.
Vendors - player owned and placeable vendors, to sell items provided by players.
Professions - character professions that add unique player-only abilities to the game.
Colleges - colleges of skills/spells, increase your power/ability with a spell/skill, by increasing your skill in the appropiate college.
MXP Protocol - Mud eXtension Protocol. Click-able objects, exits, ect.
MSP Protocol - Mud Sound Protocol. Allows the use of sound.
For builders - EXTENSIVE building enhancements to make building easier, faster and more powerful. I will refer you to the building section of the webpage.

Amitoune MUD