Check out BooksFree, a book rental service which allows you to choose a monthly plan that fits your reading habits. You can choose to have two, four, six or even 15 books out at a time, with unlimited books per month. Your monthly fee increases with the number of books out at a time.
Look into PaperBackSwap, a book sharing site that gives members one book credit for each book sent to anther reader; you can then trade in those book credits to receive books from the extensive collective library. The books you receive are yours to keep -- or re-list for other members.
Try BookMooch, where you list the books you want to give away (just type in ISBNs) and send them to people who request them. You can then request anyone else's books with the points you received for sending yours off.
See what you think of BookCrossing, a free online book club. Members of BookCrossing label their books with a BookCrossing label. Members then release the book to another member or leave the book in a public place where it can be picked up. Those who pick up the books can log into BookCrossing and relate where the book ended up. You can join BookCrossing and let others know which books you are looking for while sending your own books on a new journey.