CthulhuMUD is one of the best muds i have found and played. The races are unique, such as the deep ones live underwater and can breathe underwater. Yithians bodies are weak, but they possess other people and 'adapt' skills from them.

It is full of awesome features such as The Dreamlands, where you can get to if you sleep and dream. The need to eat and drink also adds some gameplay, so you have to earn money and be active. It is Roleplay encouraged but not enforced, so it is good for everyone! It is a full PK server, but there has to be a reason.

The only downfall is the small amount of players - so you can fix that by joining CthulhuMUD! You don't have to know the books either to play.