The website Where’s George lets people enter their dollar bills and see where bills have traveled. I always thought it was an interesting idea. For example, check out this example of a dollar bill trace. You can even see a map where the bill has been.
Lately, however, a surprising use was found for Where’s George. From this article: “…scientists have unveiled statistical laws of human travel in the United States, and developed a mathematical description that can be used to model the spread of infectious disease in this country.”
(via Slashdot)