BookHopper is an international site for people who want to swap books. After registering, you list the books you want to swap (searching via title, author or ISBN), and wait for people to request them. When they do, you receive an email with their address (not a service for the internet paranoid, I guess) and a direct email for contact if needed, and then you send on the book at your own cost through the post.

In return, you get to take a certain amount of books for yourself - for every 10 books you list, you can take 1 book every 3 weeks/for every 20 you can take 1 every 2 weeks/for 30 or above you can take 1 every week. It's working surprisingly well for me, I'm sending out my second book tomorrow and one new one is winging it's way towards me at the moment (topic: samurai and cowboys!). There's absolutely loads of Terry Pratchett available, including my copy of Thud!, so I'll hopefully get around to reading more of his work in the next 6 months.

And because it's international, once I become one of those American Immigrants and start collecting too many paperbacks there, I'll re-register to the American side and start swapping again (despite it being a world service, swapping is kept local to save expensive shipping rates/impact on the environment). I don't mind paying for postage, it's a hell of a lot easier and human than carting boxes of books to a charity shop/used book shop to be given to strangers. Doing it this way makes me feel like I'm sending my babies to people who will care for them + getting free books in return! Yay internet!