What is Second Life?
Second Life is an online 3D virtual world multi-user, invented in the 2003 by the American company Linden Lab. The system provides its users (called "Residents") the tools to add and create in the "virtual world" of Second Life new graphical contents: objects, background, faces of the characters, audiovisual content, etc.. The peculiarity of the Second Life’s world is to give users the freedom to take advantage of the copyright for the objects they create, which can be sold and traded between "resident" using a virtual currency (the Linden Dollar) that can be converted into real American dollars.
At the end of March 2008, approximately 13 million accounts were registered, although a large percentage of these are inactive, some Residents have multiple accounts, and there are no reliable figures for actual long term consistent usage. In January 2008, residents spent 28,274,505 hours there, so on average about 38,000 residents were logged on at any particular moment. Despite its prominence, Second Life has notable competitors, including IMVU, There, Active Worlds, Kaneva, and the erotic-oriented Red Light Center.
http://secondlife.com [2 type of registration, FREE (no fee, and you have to earn the money you'll spend) and Premium ($10 at month, you already have money to spend)]