I spent _so_ many hours as a kid on RetroMUD. Nothing else will improve your typing and console scanning skills like being a healer during a massive boss fight.
Retromud is the only MUD I've ever played as well. Started in 2001 or 2002 and every once in a while will go through phases where I play again.
If you aren't that into Role Playing, and you prefer team based PvE combat, I think Retromud is the one to check out. 20 primary guilds (classes) and 80+ races, as well as branching paths from each of the primary guilds makes for huge number of possibilities for your character. Though to start out I'd suggest a Dragonian Fallen.
http://www.retromud.org/