Second Life, but hardly limited to it -- witness, for example, the real-world price of EVE Online's ships. Or, take David Storey, alias Deathifier, alias "that guy who spent a whole lot of money on Planet Calypso items in auctions." And he's spent quite a lot of money -- in the game world, he owns both an island and a staggeringly expensive egg with a yet-undisclosed purchase.
So why did he buy it? As he puts it in a recent interview available on Forbes.com, the same reason people make any expensive purchase -- because it makes you feel good. According to the interview, it's not as if the investment has been a poor business decision. The virtual island he bought in Planet Calypso for $26,500 brings in roughly $100,000 a year, as he uses it for a rare game preserve and taxes local hunters. The full piece has more information on his outlook and business practices, as well as a brief overview of the markets of virtual worlds and MMOs in general.