Blizzard recently introduced a new item into their video game, World of Warcraft, that allows player to use and gain 30 days of playtime. Players may buy these tokens for $14.99 (the cost of 30 days playtime) or ~21,000 gold (in-game currency). This item and the way that Blizzard has made sure to keep the game balanced with it has worked out pretty well from what I have seen.
Real World Trading (RWT) is a system where players in a video game exchange virtual items in-game for real world assets, typically money. This system has looked down upon by many players and video game companies for years now. So all a player has to do is simply buy a token for $15, then proceed to sell the token in game for ~20,000 gold. This is a good system for those both ends of this deal. For Blizzard, they sell a month of time in game for $15, like they always have. People can buy these tokens in game and get to play for free (minus their virtual money). So Blizzard is losing no money during all of this and players now have an alternative method of playing. Overall, I like how this is balanced and well thought out on Blizzard's end of the deal.