I just recently discovered this service and, wow, is it awesome (especially for homeschoolers and/or other avid readers)!  You list books (they don’t HAVE to be paperback, but most are) that you have and are willing to pass on.  When someone else wants one that you have, they request it from you, and you mail it to them at your expense (usually about $1.60).  For each book that you send, you earn one credit, which can be used to request someone ELSE’S book, which they send you at THEIR expense.  When you post your first nine books, you get a free three book credits.

I thought for sure no one would want what I have and/or no one would have what I want, but I have been pleasantly surprised.

[UPDATE 11/2013: I have received more than 220 books through PaperBackSwap, to date, and they estimate I’ve saved at least $435.  I’d say that’s a very conservative estimate.  And these are often harder-to-find books.]