It is called SwapTree. The idea is that you list items you have, then swap with those who have what you want. I like that it encourages utilizing what you already have - I'm not sure that shipping items all across the country is as great of an idea as it could be. For example, a local swap would be better than the national swap, but it is a very good place to start.
Craiglist, which has both a "barter" and a "free" section, is also nice, as well, as locally here in State College there is State College Classifieds.
Please comment with other good sites for trading, borrowing, selling used stuff (aside from the well known amazon, ebay, etc.)