I don't know how I've missed this over the past year, but Second Life is a very interesting "game". I put the word game in quotes, because it is a simulation and an environment more than it is an actual game.
The system is designed as a completely user modifiable environment where users can construct items and places and other users can experience them. Of course, users can interact via instant messaging and location-based chat, but there is also an advanced economic model that allows users to create and produce items and spaces and provide each with actions, giving the user a real opportunity to create a valuable addition to the game and "profit" from it.
The economic system levies taxes on structures and items on a monthly basis, but users receive a stipend based on the response of other players to their gameplay, avatar design, and usefulness of their creations. Further, users can sell items to each other, either in real time or in a shop-type environment.
Unfortunately, the game is only for Windows right now. Fortunately, the Macintosh version is underway and if you know of any interested Macintosh programmers in the Bay area, they are looking for people.