But I like Paperbackswap much MUUUCH better. and SwapaDVD and SwapaCD. They give you the option of printing postage, so even if it over 13 oz, you can still mail it from home; without the trip to the post office.
I love books. So, somehow, through the magical powers of the interwebs, I discovered a site called BookMooch.
Now, I can get books I've been looking all over the place for in exchange for one of my old dustys, and...well, and the shipping for sending them out.