In what is probably the most impressive creative project in Second Life since the giant penis sculptures of SL retail, a group of players have used the game’s building and scripting tools to create an in-game cyberpunk MMO. So, yeah, that’s an MMO made inside of an MMO.
Remnants of Earth [official site] has players take on the role of a hero who has been left behind on an abandoned Earth. Despite the MMO nomenclature, this is essentially a fully-featured table-top RPG. In this game-within-a-game players must interact and role-play with one another through scripted events. You can also do things like crafting and go on quests.