Bookcrossing is the act of leaving books in places for other people to find and read them. It's like a random a book exchange. With the site, you can register books, and track them if the people who pick them up report finding them. Its neat. I have crossed a few books, but since I had to pare down my collection, I don't have a lot of stuff I'm willing to part with right now. And I don't have the time to go and track down other people's crossed books, either :o Still, I like the idea.