You can buy Linden dollars with U.S. dollars or earn them in the game.

FORBES

"Second Life" balances are recorded in a special currency called Linden dollars.

FORBES: Taxing Virtual Worlds

Second Life -- a 3-D environment that looks kind of like a video game -- has a real economy based on "Linden Dollars, " which can be exchanged for major world currencies on a market system.

CNN: Can people actually 'own' virtual land?

And while some virtual worlds have discouraged the exchange of game money for dollars, Linden Labs condones the practice.

FORBES: Second Life Goes Legit

People pay real dollars to Linden Lab for access to virtual land.

CNN: Can people actually 'own' virtual land?