I read today about Second Life, which sounds really interesting. It is “a 3D online persistent space totally created and evolved by its users”. Similar to Project Entropia, it has an in-game currency which can (at least in theory) be easily converted to real-world currency. Project Entropia really plays up this aspect, luring you to try to earn money by playing their game. Second Life seems more directed at designers and content creators, but they also promise the ability to “make real money in a virtual world”.
You can create one avatar in Second Life for free, but additional accounts (and premium accounts, which allow access to land ownership and a higher weekly allowance) cost $10 per month.