MUD. Multi-User-Dungeon. Text-based roleplaying, all you need is telnet (well... if you don't use windows. The windows telnet is just crappy. Better uses PuTTY or something like that) and a big amount of fantasy, then you connect to the MUD, log in (or for the first time, create a character) and then start with your quests, explore the world, do good (or bad) deeds, interact with other online users (depending on the MUD and the time you log in, sometimes up to 200 or more useres online at the same time), and so on.
