Seductively simple idea, drawing on a plethora of sites such as Zipcar and Swap. Although many of these are yet to spread outside London in the UK, the potential is clear; especially in a period of economic hardship. I guess the nearest I've ever got to this is offloading stuff on Realcycle/Freecycle - although that's basically a one-way affair.

Most interesting perhaps is the prospect of peer-to-peer (P2P) sharing; for example, renting your vehicle (which spends most of its time unused) to someone else. The hurdle is the eBay-style reputation that will be needed to make P2P lending work. Building up a reputation buying and selling low-value items online is a lot more straightforward than through lending and borrowing cars etc...