Second Life is heading this direction (Score 1)

Linden Labs, the creators of Second Life, have said that their hope is to eventually make the technology behind their virtual world more of an open standard, thus allowing the metaverse they've created to grow exponentially. Coupled with SL's basic premise of user created content, it's not inconceivable that in the future sophisticated MMOs could be built on top of Second Life by third party game companies.