I've been using Bookmooch for a few months now, and I can recommend it highly. It's an amazingly simple idea: list the books you want to give away, and the books you want, and the site matches givers to receivers. (I've seen it called "Bittorrent for books.") The only cost is postage for the books you send, but hey, you get free books in return so no complaints here.
When you first sign up, you have to add ten books to your give-away inventory. That gives you one point to request a book, and from then on you get a point whenever you give a book away. Receiving a book costs one point. Easy. I've more or less added my whole Amazon wish list to Bookmooch, and once in a while I get an e-mail telling me that a book I want has become available. One click and a week or so later it's in my mailbox.
My wife and I are on a slow, painful de-cluttering binge at home, and this is how I'm weeding my bookshelves. (I know, I'm getting books in return to take up shelf space again, but at least they're books I'm sure I want to keep. And I'm sending out at least as many as I'm acquiring.)