Some of my MUD accounts from 1991-1992, which I still occasionally log into from time to time, out of morbid curiousity. They're inexplicably still online, but effectively ghost towns.
Other than that, probably my Gmail account from the beta days.
MultiUserDomain - a precursor to today's MMO. You could connect to an ascii based adventure/roleplaying game that others could connect as well and communicate. I played astaria.net back in the late 90s.
Multi-User Dungeon; Cf. MUCK for multi-user chat kingdom and friends. Essentially a shared text-only set of chat rooms with a theme and some built-in game logic.
Other than that, probably my Gmail account from the beta days.