Most of us who profess anarchy are unlikely to have a clear memory of a time when currency was still "backed"- on a standard. When currency had a value other than what someone tells us it has. And even those of us that might remember the gold-standard (I'm not one of them), are unlikely to have ever converted our currency to gold or silver. Though I do have a gold reserve note someplace. Moreover, not one of us has lived in a world where currency was not monopolized by a government.
In an anarchy, there would be no government minting currency, however, this does not prohibit people from minting their own currency. It does prohibit a floating currency though- because two things give currency value: material backing, or faith in its producer. Since "faith in the producer" is roughly equivalent to accepting IOUs, we can assume that faith-based currency will never take hold, unless there's a powerful institution to enforce it.
So we need to look at backed currencies. When backing a currency, there are a few things you need to consider. First, the backing item must be something for which there is high demand and low supply- it must have value. Secondly, it must be something that is useless. Gold used to be a great standard- until we discovered millions of industrial uses for it. At this point, I'm not sure that there's any rare item, in demand, that doesn't have an industrial purpose. Some search my be required to find one.
It must be purposeless. I at one point was espousing "wheat-bucks" and "flax-script", etc. but manycolored pointed something out- what if someone wants to affect the value of the currency? They just have to destroy the backing material. Imagine if someone wanted to increase the value of their wheat-bucks, and razed thousands of acres of farm to the ground. That's very undergood.
Currency is important, even in an anarchy, and even in a communo-anarchy. Face it, in a communo-anarchy, no one is going to be able to force you to share, except social pressure. But you can't count on that, so you need some method of ensuring that someone will be willing to surrender item a, in order to gain item b. Someone won't give up their wheat until they know that they can get that extra lot of field space to produce more. You can use barter, but that breaks down when the producer wants item b, but the consumer only has item c. Currency is an abstraction.
We agree, as a group, to take these slips of paper, and say that this slip represents x amount of some good. That good is something everyone wants, if only because it is able to purchase other goods. This ensures that the consumer always has something that the producer wants.
In toying around with currency ideas, I hit upon one that I think is interesting. Man-hours. It's a fairly elegant system- we determine a "base unit of work", something that takes an hour to do, but requires no real skill or training. Then, for people to accrue currency, they must invest their own man-hours into labor. For doctors, they must have a high degree of training, skill, committment, etc. so we could say that a doctor earns 4*BUW(base unit of work) man-hours per hour. A burger flipper probably is earning 1.3BUW/hour. And so on.
This is a currency unit that can work in an anarchy, and accurately represents the production of goods. This widget required ten-man-hours to get from the factory floor to the consumer.
Just some ramblings anyway...