The idea of online economies has fascinated me. You put together an online multiplayer RPG, and economic forces are soon to follow. Trade, inflation, the works. People start selling in-game goods at places like eBay. Some even manage to make a living at it. Most games frown on this type of activity, and explicitly ban sale of in-game goods for real world money.
Cnet has an article about two such games called Entropia and Second Life, among the first to actively embrace their in-game/real world economy.
I’m not sure why these mini economies are so interesting to me. Maybe because they’re a microcosm; you can watch forces which are slow in the real world play themselves out on a manageable time scale.