There are two popular MUSH servers dedicated to the works of J.R.R. Tolkien:
Elendor is the oldest and most popular Tolkien-based MUSH, based on Third AgeMiddle-earth with the timeframe set before the quest of the Ring. Knowledge of the The Lord of the Rings trilogy is required.
Beleriand is based on Middle-earth in the First Age, after the exile of the Noldor and the establishment of elven cultures in Middle-earth. The Silmarillion is the primary reading material for players.