In many online video games, like World of Warcraft or EVE, players can have their characters trade for new abilities or weapons or all sorts of things. They acquire currency within the game to come up with the means to acquire these add-ons. And if they don't have the time or inclination to perform these more menial tasks, they can trade with other players for the currency. This created an incentive for 'gold farmers' to sell their game currency to other players for real cash on websites outside of the game environment. There are thousands of 'players' in less developed countries who earn a modest living selling the 'gold' they farm. Game publishers generally discourage this and so it became a 'black market' activity moderately suppressed by the game publisher.
In its recent release, the game Diablo III will embraced this activity and even expects to generate earnings by taking a percentage of each transaction. A nice discussion of this can be found at Penny Arcade's video of this market place.
Hat Tip: A Ward