Project Entropia

"In Project Entropia you will able to enter a whole world with amazing three-dimensional environments using a computer and the internet. It will be a massive virtual world where millions of users can interact with each other at the same time. Project Entropia will have a real economy system that allows you as a user to exchange real life money into PED (Project Entropia Dollars) and then back into a real currency again."

Free, with option to pay for additional features.