Since February 2009 I've been a member of Bookmooch. Bookmooch is a book trading site. Basically you post books you don't want any more and mail them to anyone who asks for them. For each book you post you get 1/10th of a point, for every book you mail you get 1 point and for each book you mark as received you get 1/10th of a point. You get extra points for mailing internationally.
Since I've started I've given 56 books and received 69 books with enough credits left so that I could get 17 more books. Depending on you tastes the selection might not be the best, but I've got odd tastes which have kept me busy trading.
After all those trades I haven't had any bad experiences. People on Bookmooch seem friendly and generous. If you have old books you don't want any more give it a try, it beats tossing out the books.
It's gotten to the point where I go to the book sales at the local library. On the last day of the sales they sell books by the bag at a low price. I pick up a lot of random stuff, usually the just picking the books that are left and in the best condition. I then post them on Bookmooch so they can go to people that want them and I get some credits for them. Sure I'm paying for all that postage, but I'm finding all sorts of books I probalby never would have purchased just browsing on Amazon.