Of all the things that the internet enables, I think one of most interesting is new currency. Traditionally the purview of nation-states, I'm not sure what the rise of new currency means. Fundamentally money is a signal, so something important is going on here.

Mochi Coins
Gamers who buy MochiCoins can spend them in any game that uses the Coins API, anywhere on the web.
L$
Once you go inworld, you'll may wish to explore shopping sites or other services that ask for payment of a Linden™ dollar. This is the official unit of trade in the Second Life virtual world.
BBE Scrip
Bartering is an excellent way to expand your business. BBE will refer customers to you. The barter dollars you earn on these referrals can be used to purchase basic cash expenses, reducing the cash flowing out of your business each month.

Curreny in the U.S has a varied and colorful history.

Update: Follow back links and such I just discovered the OpenSocial Virtual Currency Spec Proposal, which was heretofore unknown to me.