The Swap Meet
Post books you'd like to exchange on BookMooch.com, and whenever one of 100,000-plus "moochers" requests one, you'll earn a point for mailing it. Redeem each point for any of the 300,000-plus titles in the BookMooch inventory—all you pay for is the postage.
The Postal Service
Trade hardcovers and audiobooks as well as softcovers at PaperBackSwap.com, and get updates on newly available titles via their daily e-mail digest. Stroke of genius: the online postage program, which allows you to print the postage on a mailing label directly from your computer.
The Deep End
BookSwim.com lets you organize a "pool" of titles you'd like to rent (just like a Netflix queue); for $24 to $60 per month, you'll receive three to eleven at a time. Shipping is included in the subscription fee, and you can purchase any BookSwim rental at the click of a button.
The Good Read
With more than two million titles at bargain prices, BetterWorld.com donates a portion of revenues to literacy causes such as Books for Africa and Worldfund. Shipping is free within the U.S., and the site minimizes its carbon footprint through an offset program with CarbonFund.org.