I use a site called wheresgeorge.com. I enter the serial numbers from my paper money, spend it, and when the next person enters the serial number from that bill, an e-mail is sent to me, and I can go to the site and see where the bill wound up. I was notified today about a $10 bill I entered more than three years ago that has had an interesting route. Take a look. My entry and comment are at the bottom. The latest entry is at the top.