If you enjoy reading books on a Kindle or Nook, check out LendInk. It is a free service which matches people who want to borrow e-books with people who already own them. Once you sign up, you browse the library of available titles. The limit is three books borrowed at any one time. A book may be borrowed for up to 14 days.
I have not used this service myself. However, for those who are avid readers and do their reading on a Kindle or Nook, this service may become a significant part of your reading experience.