There’s another MMORPG in the same vein as Second Life that’s built on a real cash economy: Entropia Universe. formerly known as Project Entropia. This game isn’t a pure social world as there are actual gameplay elements (e. g. hunting, missions, crafting, etc.) so it does feel more like a traditional MMORPG than Second Life.
The currency is called Project Entropia Dollars (PED) and exists at a fixed exchange rate of 10 PED per $1 USD. What’s unique about Entropia Universe is that PED can be directly withdrawn into a bank account – no middleman necessary. In a very real way, Entropia Universe is the closest thing you’ll find to a game whose economy is directly related to actual money.