Paperback Swap is the coolest - you can swap books (not just paperbacks) with people across the United States and the most it'll cost you is the cost of shipping. Basically, you post books you're willing to swap and if someone requests one of your books, you mail it to the person. In return, when you request someone else's book, they pay the shipping to mail it to you. I can't wait to try it!
UPDATE: I posted about Paperback Swap more than six months ago, and now that I've used it, I can't stop raving about this free site. I got a bunch of friends hooked on it, too, and they all love it. I've swapped so many books and have never had any problems. I'm bumping this up for readers who may have missed my original post about it because this is one of those good things that's worth spreading the word about. Just post 10 books to start that you're willing to swap, and you get two credits automatically - that's two free books!