Barter
From the Wikipedia article at http://en.wikipedia.org/wiki/Barter
"Barter is a type of trade that doesn't use any medium of exchange, in which goods or services are exchanged for other goods and/or services. It can be bilateral or multilateral as trade.
Barter and money have different means of balancing an economic exchange. Barter is used in societies where no monetary system exists. When there is one, it is also used, especially in economies suffering from a very unstable currency." (http://en.wikipedia.org/wiki/Barter)
Examples
- BarterQuest, http://www.u-exchange.com/
- Swap It Now, http://www.swap-it-now.com/
- SwapTreasures, http://www.swaptreasures.com/
- U-Exchange, http://www.u-exchange.com/
Tools
There is a whole ecosystem of modules and software which support bartering mechanisms: