If you had to guess, how many bedrooms would you say are going spare right now in homes across Australia? How many perfectly good rooms are being used just for storing disused dumb-bells and dusty box sets of DVDs?

The latest census says the answer is about nine million, more than one for every three Australians. That's nine million spare rooms which could be put to more productive use if only there was some way to match up people who own rooms with people who'd like to stay in them.

Airbnb: One of a host of new services that link people with stuff they're not using to those willing to pay for it. Credit:Bloomberg

As it turns out, there is. Airbnb is one of a host of new "sharing economy" services that link people who own stuff they're not using with those willing to pay to do so.

Other examples include Uber and Lyft – ride-sharing apps which let you catch a lift with drivers going your way – and Fon – which lets travellers connect to Wi-Fi networks anywhere in the world in exchange for some of their own bandwidth at home.