Gary Hayes has put together a really nice tour of virtual worlds. You should really visit his overview, since he has some good analysis as well:

• That there are quite a few worlds now getting their balance on the shoulders of Second Life and really getting to grips with the social networking aspects vs the 3D’ness
• There IS a balance between a social space and an ‘agreed’ advertorial world – “you give me valid experience, I accept a level of advertising”

I have taken the liberty of embedding his video below. You’ll see Twinity in there in the first half.