i second the UO association because of the concept of virtual real estate. In UO housing was a large aspect, regulated with an ownership system, which made the entire world nuanced and relevant. Beyond second-life, I have not seen that level of world-building in any game. Even Minecraft on a server with block area ownership is not the same, as you typically just end up with an endless landscape of art pieces, rather than a functional user-created environment.

That's pretty much what EVE Online is. Most of the galaxy belongs to player-owned corporate alliances, who build infrastructure such as space stations and outposts.

Before UO came out, MUSHes had exactly that (with object/room ownership and @quota to constrain what you can build at any given point).

Nothing is ever new, of course. =)