Where have I been? I just discovered BookCrossing.com, and I am in love. You read a book. You regsiter it at the bookcrossing.com site, then you leave it in a public place for someone else to find. That someone (in theory) finds the book, goes to the site & checks in, reads the book, then sets it free again to find its next reader ("Read, Release, Repeat").
Book karma. Awesome.