This is kinda cool... Bookcrossing - once you're done with a book, you go to their site and register the book and leave comments about the book. They give you a special code (called a BCID) which you write in the front cover along with the website address.
Then, you leave the book in a public place.
The theory is that someone will find the book, read it, and possibly go back to the website and register that they found it.
Sort of like a catch-and-release program to share books.
Similar to Where's George and Grafedia.