A Linden Dollar (L$) as such does not exist. It is not a real currency but a virtual one.
The Linden Dollar (L$) is the currency residents use in the Second Life world. "Linden" comes from the company behind Second Life, the Linden Lab located in San Francisco.
For buying and selling land, islands and other assets, residents need the Linden Dollar. What gave Second Life a boost in its development, was the link between virtual and real world. With the possibility to exchange US Dollars for Linden Dollars both worlds got connected.
There are certain ways to exchange US Dollars for Linden Dollars and Linden Dollars for US Dollars. The most popular one is LindeX, the official trading platform from Linden Lab.
The Linden Dollar - a virtual currency
Because a fairly stabile exchange rate is important for customers, the makers of the Linden Dollar try to establish a stabile exchange rate. Since introduction the Linden Dollar they did a good job. The exchange rate of the Linden Dollar varies just under 270 L$ per US-Dollar.
Some facts and figures:
> Volume in L$ traded June 9 2007: 62.277.289
> Square Meters Auctioned in May 2007: 22.988.144
> Gross national product of the "national economy" Second Life in 2006, calculated in Million US-Dollar: 64
> Total Residents by June 11 2007: 7.148.028
Source: Secondlife.com, 2007, Economic Statistics and own estimation