Innovation in online swapping services is still at the early stage. The swap companies are still figuring out the basic economics, such as: Should swaps be item for item, or should there be an intermediate fake currency to track the value of things? You can see radically different philosophies in the services, ranging from the points-based system of Peerflix to the pure barter system of Swaptree.
Another service also has a bit of momentum: SwapThing. I'll say up front that it has the ugliest user interface of any of the swap services, but it also has a lot of flexibility that may work to its users' advantages. For one thing, SwapThing doesn't restrict you to just swapping. If you want to get rid of an item, you can put it up for swap, cash, or either.
Also, SwapThing is the first barter site I've seen that explicitly supports the trading of services. So if you're a poor consultant who wants to trade your time for an item on the network, SwapThing will let you barter your expertise directly. Small-business owners might find this useful. As CEO Jessica Hardwick rightly notes, "It's rare to meet a service provider that doesn't have downtime." This gives the downtime more potential value.
Here's another swap site I stumbled across (literally--I was using StumbleUpon): Zunafish. This one is like-for-like only: you can trade DVDs only for other DVDs, books for other books, and so on. That to me, drastically reduces the value of a swapping network. However, Zunafish has a much better user interface than SwapThing, and that counts for something. You're unlikely to enter items into a swapping service if it's a drag to do so, and on Zunafish it's easy.