I came across Bookcrossing a while ago. It is a nice idea. The Guardian describes it like this:
The concept is finders-keepers meets interactive virtual lending library. The rules are simple. First take a book down from your shelf. It should be one you love. (Ideally, if you ruled the world you would make reading of this book compulsory.) Log onto bookcrossing.com and register. Print out a label and a number for your book. Release it into the wild. The person who finds the book will see the invitation to the website where they can log their find, eventually write a review and then rerelease the book themselves. In theory, as the book travels around, it should build up an online profile of reviews. [Guardian Unlimited Books | By genre | Finders, readers]
Libraries would be good crossing places?