Back in March, we surveyed you (over in D.C.) to see if you used an online book, CD, or DVD swapping site. From your comments and not surprisingly, it appears that the most popular thing to swap is books. We've considered this too, and with all of your suggestions there are plenty of options out there. For those of you new yet interested in book swapping, here's a great starting point roundup. Add to our list if you know of others!
The process for each seems very straightforward: list books you want to give away, receive requests, mail your books (shipping is typically low), and search for books you want to receive.
Here's a starter list of the sites you recommended:
Image: Colorful Book Nooks