BookMooch

A quick note to sing the praises of BookMooch. You're unlikely to find any books for free on there that are intensely rare or valuable, but if you don't mind the odd trip to the post office it's a really satisfying experience to send books you don't want to others around the world who want them more. And it's fun to obtain titles from others that you would otherwise have had to pay for. It could evolve into a big global library service, and is a nice example of green economics/freecycling in action.