BookCrossing: Another kind of book swap. You tag one of your books with a URL and a unique ID, then “set it free” by leaving it where a stranger will find it. Hopefully the finder logs onto BookCrossing to make a journal entry, then sets the book free again.
Here’s a good article about BookCrossings goings-on in the Bay Area from the SF Chronicle. I’d seen Where’s George, but hadn’t seen PhotoTag — that’s cool, too. :)
Also related: Geocaching.