"Second Life is a 3-D virtual social networking world entirely built and owned by its residents. Since opening to the public in 2003, it has grown explosively and today is inhabited by a total of 6,240,591 people from around the globe." Second Life allows each user to create his or her own avatar, offering a large degree of freedom in choosing one's own appearance. As a result, most Second Life users' avatars differ significantly in looks from their real-life operators (for example, Jason, in Second Life, has green skin), and only very few create their Second Life avatars as a near exact representation of their real selves (like Henry Jenkins).
Second Life is unlike any other MMO because the goal or function of Second Life is entirely user-determined. As a result, the world of Second Life has been built from the bottom up to cater to almost anything a Second Life user could want, from creating original art and attending lectures to greed and lust. Some people and companies use Second Life as a commercial tool. Many large businesses (Reebok, Apple, and Nike, for example) have established headquarters in Second Life that advertise their real life products.
http://www.secondlife.com/ Second Life Homepage