My favorite is
http://www.retromud.org/ . Not much for PVPers outside of occasional no-death-penalty war arenas, no RP enforced at all, but a whole ton of races and classes, more than just about any other mud out there. My characters include a werewolf abjurer (spiritual protection path, with minor Aikido skills too, not much healing or damage at all) who can also bite the heck out of people, and is also a shaman with a lot of neat flavor spells, transformation stuffs... And a satyr mage girl who's currently a geomancer with a little bit of telemancer (disintegration and stone damage spells, gemcutting, slinging skills, etc) but who's shortly reincarnating to a lightning-based path (flight, interrupts) and taking some levels in bard and druid (druid so she can still use her sling).
It's heavily party-based. You CAN solo a bit, especially at low levels, but you really want to get at least two other party members for most interesting stuff, up to eight people. Some classes are specialized, others not, some are very high-tension and some are way more relaxed.
You've probably seen just as many banner ads for Retro as you have for all the Iron Realms games...
It's a lot of fun to explore new areas you've never been in before. And there's a lot. :)