At BookCrossing you can register a book, and then "release" the book (leave it in a public place) for a passerby to pick up. The "catcher" (finder) can go to the BookCrossing Website and leave notes about the book, then re-release the book so the process can be continued. The goal is to track the progress of the book as it moves from person to person.
Today I released a book. I left it on a bench at a grocery store, and I'll wait to see if someone picks it up and adds a journal entry on BookCrossing.