I saw this website plugged in our Metro (commuter freebie newspaper) this week, I think as part of Earth Day etc. It's a site where you can get rid of your unwanted books, CDs and DVDs, but instead of throwing them away you can trade them. What you do is enter the ISBN on the site, briefly describe the condition, and if someone has the item on their "want" list you'll be notified of a possible trade. You can also set up your own want list and you'll be notified the same way if someone offers an item you want.
Seems like it would be a great idea to thin out the shelves of stuff you haven't looked at in years and maybe find something for it in return.
I have no connection to the company etc. except signing up.
Looks like it's US-only at least for the time being, most likely to avoid complications with postage.