Stephen Hill, on the pho list, points to a great interview with Bernard Lietaer on complementary currencies (ignore the rather schmaltzy new age design; this is deep stuff about monetary theory, clearly explained).
First, let's define what a currency is, because most textbooks don't teach what money is. They only explain its functions, that is, what money does. I define money, or currency, as an agreement within a community to use something as a medium of exchange. It's therefore not a thing, it's only an agreement—like a marriage, like a political party, like a business deal. And most of the time, it's done unconsciously. Nobody's polled about whether you want to use dollars. We're living in this money world like fish in water, taking it completely for granted.
Now the point is: there are many new agreements being made within communities as to the kind of medium of exchange they are willing to accept. As I said, in Britain, you can use frequent flier miles as currency. It's not a universal currency, it's not legal tender, but you can go to the supermarket and buy stuff. And in the United States, it's just a question of time before privately issued currencies will be used to make purchases. Even Alan Greenspan, the governor of the Federal Reserve and the official guardian of the conventional money system, says, "We will see a return of private currencies in the 21st century."
I think Heinlein once wrote that money should be an adjective, not a verb, that we should talk bout the moneyness of things.
Company Stock is in effect private currency, with a fluctuating exchange rate tracked by the Stock Market, and much of the net runs on non-monetary exchange.
Bridging these complementary currencies into the dollar economy is an interesting challenge.