secondlife
is a massively multiplayer game with user generated content.
You pay.
There is a demo or trial account period.
People pay for things like land, buildings, characters, animations, etc.
There is a whole economy within the game and in the real world based on content for the game.
There are a lot of books on the subject.
Google has 136 million hits for it
The export as far as I can tell is built into AC3D.
Details here