This is the third time I've found a $1 dollar bill (portrait of George Washington) stamped by the site wheresgeorge.com. When I find one, I just go to the site and enter identification information. Then I can see where the dollar bill has been and keep track of where it going when it leaves my hands. Here's the map for the dollar bill I have now.
There is a sister site for Canadians called whereswilly.com. The FAQ had this in case you wondered who "Willy" was:
'"Willy" is Sir Wilfrid Laurier - The first French Canadian Prime Minister. He held office from 1896 to 1911. His portrait is on the Canadian $5 bill.'