At PaperBackSwap.com, you can trade books you've read for ones you want-for no more than the cost of postage.

HOW IT WORKS Become a member(it's free), then list books you'd like to offload. The site notifies you when another member wants your book; you mail it with a label you print from home. ( Most books cost $2.23 to ship). Then you get a credit you can use to order any book on the site.

WHAT'S AVAILABLE Inventory's always changing, but right now, "we have half a million unique titles, including hardbacks and audio," says Richard Pickering, who founded the site when used-book stores didn't want his old airport paperbacks.

HOW IT'S DOING "In the last three months, we've added 500 members a day," says Pickering. "We're growing like gangbusters!"