I noticed that a dollar bill of mine had red stamps on both sides that said "Track This Bill [url=http://www.wheresgeorge.com" onclick="window.open(this.href);return false;]www.wheresgeorge.com[/url]".
I went to that site and once you enter the bill's serial number it shows a history of where the bill has been. There wasn't much info about mine, other that it was in Puyallup, WA four years ago.
Here is an example of one with a more detailed history...
