wow.. I'm surprised no one mentioned DSL (http://www.dsl-mud.org) it's a fun rp-enforced mud, I say enforced because the implementor, scorn, is a nazi, but we love him anyway. It's got about 250-300 actives, each with a few alts. It's not really newbie friendly from the code, but the people tend to help you out. this is a shameless plug, after all even though i'm stuck with playing it in black and white due to the Ansii fark, i still play it..