PaperBackSwap.com lets you trade books you no longer want for books you do want. Similar to BookMooch, PBS works on a point system: Every time you mail out a book, you earn a credit. Every time you receive a book, a credit is deducted. In other words, you have to send books to get books.

Unlike many other trading services, PBS doesn't charge you for each "transaction." The only thing you pay is postage, and the site ingeniously produces a printable mailer that shows the exact Media Mail rate for the book you're sending. Slap on the right number of stamps and drop it in the mailbox — no trip to the post office required.