Through murmurings on a message board, I recently discovered something so great I have to share it here: Paperbackswap.com. The premise is fairly simple--you join up, then list books you have (don't have to be paperbacks, can also be hardcovers or even audio) that are in good shape, but you want to get rid of. These books get listed on the site, and when someone wants one of them, the system emails you and lets you know. You send it to them them. When they get it, they log in and tell the system, and then you are issued a credit, which you can use to order another book from someone else. Simple enough. As a bonus, you get three credits when you list your first nine or more books, even before anybody requests any of them.
It's fantastic! Media mail shipping isn't expensive (usually $1.59 for a regular sized paperback), so you can fairly cheaply get rid of your old reading material and get new stuff. The selection is good, if not great. If you were a genre fiction or series reader (can you feel me judging you?), it would be really great--the site is full of Harlequin romances and those westernish Christianish series. But even if you're more like a me, a just-about-anything reader with a focus on Oprah's book club type novels (can I feel you judging me now?), there's a lot to choose from. Plus it has a feature where you can "wish list" books you are looking for, then if anyone adds them, the system gives you right of first refusal on them. Excellent.
Also, if you happen to have active readers for children (more young adult than little kids), I think it would be great to get them into it. The site is full of the type of kids' books I consumed like candy when between the ages of about 8 and 13. Babysitters Club, Lemony Snickett, Harry Potter, Judy Blume, Lois Lowry...it would be great for a kid to have an account all his/her own, to swap books via U.S. Mail with others like him/her all over the country. I would have LOVED that.
I don't think there is any bonus for signing up new members, so you don't have to tell 'em I sent you, but if you're a reader who isn't 100% library (I know, I know...), definitely check it out.