I'm not positive where I initially heard about this, but I think it was Steph G. But in the past month I signed up for paperbackswap.com and I am hooked. I offloaded several books that I was through with and in turn, have received several other books that I was considering buying. They have rules about the condition the books must be in, so that people make sure they're not mailing out junk. So far, what I have received has been in pristine condition. I'll read them a few times, then swap again. I have also discovered several books on there on my homeschooling wish list for my kids, so I'm excited at the incredible amount of savings I'm having on homeschooling books.
As a side note, I only search for swap books that I can't find at the library for free*