This one comes from a mailing list I'm on. BookCrossing.com. It is kind of like Where's George for books. The idea is that you enter a book into their system and label the book with the BCID (BookCrossing ID number). Then give it to a friend, or leave it in a coffeeshop, or donate it to a charity. When someone finds the book they are supposed to go to the web site and make an entry that they found the book. One page I found interesting was their status page, which lists books that have been recently released or caught, and some other stats. My favorite is most travelled books:

1. The Canterville Ghost by Oscar Wilde
2. The Gift of the Magi by O. Henry
3. The Paradigm of Uncertainty by Lori
4. Anne of Green Gables by Lucy Maud Montgomery
5. The Legend of Sleepy Hollow by Washington Irving

Personally I'd have a somewhat difficult time doing this because I don't like giving away books I've read. Well, unless they sucked, in which case I don't care too much. I should look around my apartment and see if anything qualifies.