My current fascination is a Star Trek simulation called Beyond TrekMOO It is a world where Klingons, Ferengis, Federation, and unaligned players roleplay their way through adventures. Stop in at Oolon's Omnivorium, my restaurant there!
Another favorite playfield is Moosehead MUD, a Multiple User Dungeon here on Eskimo North. To reach Moosehead you need to telnet to sled.moosehead.com 4000. If you mis-spent your youth playing Dungeons and Dragons - then you will especially enjoy playing in a MUD. There are monsters to fight, quests to join, dungeons to explore, wizard's castles to plunder, and other players to meet and interact with. You start out at a level where a large rat or mongrel dog will give you an even fight, but eventually you will gain enough powers, skills, and hitpoints to face a dragon single handedly! MUDs usually emphasize a hack and slash mentality while the MOOs have very little combat and much more interactive roleplaying between players.
Possibly the greatest multi-player game of all time is Diplomacy. In the basic 7 player games each player heads one of the countries in Europe prior to World War I. Through negotiation (and lying is definitely a big part of the game...) you try to gain control of Europe. Originally a boardgame, Diplomacy has moved to the internet in a big way. Check it out!
And for just passing time check out Zarf's List of games.