In December, a report by the City of London Corporation suggested that barter could offer an alternative credit stream as banks rebuild their balance sheets. Indeed, the UK's fastest growing media company, Miroma, works exclusively with ad firms to allow businesses to barter for advertising.

The Strohalm Foundation, based in the Netherlands, offers open-source software to anybody wishing to set up a currency, and has helped many organisations and groups to do that in several countries. There are various methods by which the foundation has got schemes going. This often involved selling complementary currency notes at a discount: consumers buy '€1' in funny money for just 90c in real money - in effect, a 10% discount in local shops. The real money has to be saved in a bank, in case there's a run on the funny money, but there's already a 10% increase in the amount of spending power in circulation, and in the early stages having the currency 'backed' by real money gives people confidence.

The big problem, says the foundation's Olaf Veerman, is gaining critical mass: networks need to contain a variety of businesses, so that members have a wide range of options for spending their currency. "It's a challenge," he concedes. "It's the chicken/egg story."

One way around this would be for pre-existing networks - a chamber of commerce, say - to set up a currency. Governments are wary, because it was money created by German businesses that led to hyperinflation before the war, but they could give a parallel currency credibility and stability by agreeing to accept it as payment towards tax.

Historically, some have dared to do that - with some success. After Argentina's economy collapsed in the late 90s, regional authorities issued bonds in very, very low denominations - printed to look much like ordinary cash, with a moustachioed worthy on the front. At one point in 2002, these notes, which in Buenos Aires were called patacones, accounted for an astonishing 48% of all the 'money' in circulation.

To begin with, understandably, individuals and businesses were not keen to accept patacones. The teachers' union decried them as "an act of vandalism against wages" and the rank and file walked out of classrooms. The proprietor of a cut-price shop said he would refuse them because his suppliers would laugh at him if he tried to pass them on. But a Volkswagen dealership put a sign up saying it would accept them, the electricity company announced that customers could pay 60% of their bills in the new money and McDonald's even devised a special meal, the Patacombo. Carrefour, the French-owned supermarket giant, initially refused to accept patacones but lost so much business to smaller shops that it was forced to capitulate after a mere four weeks.

"Patacones were a remarkable success," says a spokesman for the Irish Foundation for the Economics of Sustainability, which promotes the widespread adoption of complementary currencies as a way through the current economic turmoil. "The fact that the federal government eventually bought them all up, at the full face value, is evidence of that."

What mattered in Argentina was not who launched the patacones but the fact that the provincial, and then the federal, government agreed to accept them for the payment of taxes. So there is still an important role for policymakers.

Partly in response to the effects of Argentina's crisis, another South American government has since launched the world's first nationwide complementary currency. The Commercial Credit Circuit (known as C3) in Uruguay includes among its members the national pension fund and major suppliers of housing, utilities and internet services - ensuring the currency's widespread acceptance and rapid growth in the network. But perhaps the most important member is the government.

"One of the main jobs for government is to make value for the currency it uses," says Lietaer. "If you want to put that in negative terms, imagine that tomorrow a government decided to take pencils instead of money as payment of tax. Within weeks, the official currency would disappear - nobody would use it - and everybody would demand payment in pencils. Taxes are a way to give value to a currency."

Even when that currency is created and controlled by businesses.