[Leech]

There are quite a few, some in various levels of development like Black Sands, Lament MUD, and Shadows of Isildur - and some have been around since before I was born like Armageddon, and Harshlands (SoI is arguably the first RPI, but it's going through a transitional period(?) currently). You have small deviations, such as the MUDs with the Inquisition theme (whether The Inquisition: Legacy and The Burning Post are RPIs or not is debatable by some) and Dark Isles (under development).

To get the full measure of what most define as the 'RPI experience' I suggest Armageddon. If you want to play without much player interaction, but want to get a good feel for the type of detail presented in RPIs, I'd suggest Lament MUD - huge, fascinating, interactive world but very few players.

Good luck!