That's really a tough thing to find, because to have a large amount of exploration without repetition in the design is just, well, hard for developers to pull off. Most of the time games with big environment have them mostly generated by a program then have little tweaks here and there by the artist. MMOs do tend to have a lot of unique towns and the like, though you have to travel through a lot of boring sameness to get there.

I guess I would recommend any of the 3D Grand Theft Autos or World of Warcraft.