I used to run a MUD or two, which were pretty much text-based multiplayer RPGs, though I'm not sure we called them that 10-12 years ago. Sticking “MUD list” or similar into Google will probably find you a few.
If you want single-player, find an
Infocom (z-Machine) interpreter and load it up with copies of the original Zork games, or use Jeff's link.