I've recently joined a used-book trading/swapping site called BookMooch created by my friend John Buckman who is also the founder of the "We are not Evil!" online music label Magnatune.
You list your used books and a wishlist for books that you'd like to read, and if they match up the sender gets a 'credit' that they can use to 'mooch' a book from someone else. To get started, just list 10 used books that you are willing to let others mooch and then you can start mooching books yourself.
Unlike a related site, there is no fee for this transaction. Bookmooch makes their money off of Amazon referral fees for new books and maybe eventually sponsorship and advertising. I think the real reason why John created it is because he and his wife live half the time in London and half in Berkeley, both love books, and want something better then hauling stuff down to the used book store.
I currently have 76 books listed in my inventory -- most they are science fiction & fantasy, but there is a smattering of mystery novels and "popular" books from M's and my book club in San Francisco as well. I have 66 books listed in my wishlist.
To date I've received 23 books care of BookMooch, and I have 14 more on the way. It costs me about $2.06 each to send including postage, label and envelope. I've mooched 10 books, which even used would have cost me over $4 each. So for an outlay of $20 and throwing a book in an envelope, I've gotten at least $40 of value. Since I've actually recieved a number of hardbacks in exchange for my paperbacks, my value may be more. More importantly, I have a connection to the people who like the books that I like.
I encourage you to join if you are a heavy book reader like I am -- you can also add yourself as my 'friend' there, though it has no effect on mooching.
I'm also using it among a couple of local friends who I regularly share books with, and delivering the next time we meet, and thus saving postage ;-)