BookCrossing
I like the BookCrossing project. Basically, what you do is to take a book that you have read, register it to get a BCID (BookCrossing ID) number for it, label it and leave it on a café or somewhere. The idea is that when someone picks it up, they register at the site so you get notified about your book being found. Also, by picking up the book there’s an informal contract to be respected: that you should release the book after you’ve read it (or immediately if you’re not interested in it).
Today, I released my copy of The Nudist on the Late Shift (see here) on a restaurant while having lunch with my friends Patrik and Anders. I told them about the project, so Patrik picked up the book immediately. He promised to read it quickly and release it back into the world.