Second Life is a 3-D virtual 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 2,320,540 people from around the globe. From the moment you enter the World you'll discover a vast digital continent, teeming with people, entertainment, experiences and opportunity. Once you've explored a bit, perhaps you'll find a perfect parcel of land to build your house or business. You'll also be surrounded by the Creations of your fellow residents. Because residents retain the rights to their digital creations, they can buy, sell and trade with other residents. The Marketplace currently supports millions of US dollars in monthly transactions. This commerce is handled with the in-world currency, the Linden dollar, which can be converted to US dollars at several thriving online currency exchanges.

OSS SecondLife client from http://hg.secondlife.com/viewer-release