AstroMUD
A space-battle oriented MUD accessible through Telnet.

Castle Arcanum
Fantasy MUD you can play through regular text, Telnet, or enhanced dhtml.

MUDConnector
One of the best listings for MUD games, support, and other services. Definitely a good site to visit if you want to see all the best MUDs out there.

NannyMUD
One of the oldest MUDs still online, and with a huge following (around 3000 active players and 100 active wizards at last count.)

Necronium
Fantasy MUD accessible by either Telnet or Java (click "play now" after reading the rules.)

Prophecy
A MUD based on the fantasy writings of David Eddings. Telnet connections only.

Sneezy MUD
Client software for cool visuals, 8 classes to pick from.

Temple of Justice
An invitation MUD, where you have to find an existing player to sponsor you.

Vargon
One of the friendlier unsupervised MUD sites I visited, also colorful (the text-based version will have color).