A new website called PaperbackSwap promises to build the largest library in the world.
It’s an online book club where you list books you want to trade to get credits which you in turn use to get books from other members. You don’t pay for anything except postage – when another member requests your book, you mail it to them (~$1.50 in postage). There is currently no fee for this service, but there will be one eventually. The video and small print on the website explains it in more detail.