Started by Ron Hornbakers' software company HumankindSystems?, BookCrossing is a Web-driven book exchange. The Web site is http://www.bookcrossing.com/ .
The object is that instead of a book residing on your bookshelf after you have finished it, you release it into the wild. You merely identify the book as free and provide a link to the BookCrossing web site along with a blurb of what to do with the book. You then leave the book where someone will likely be able to find it. When someone finds it, they can (but don't have to) log into the BookCrossing Web site and say that they have it now. After they finish the book, they too release it.
You can hunt for released books by finding your place of residence and checking to see if anybody has left some books around. (Not all released books are listed though.)
See also PassTheBookOn, DistributedLibraryProject?