"Where's George?" the best-known of these sites, lists as an example, a dollar bill that was tracked from Dayton, Ohio on July 22, 2000, through 12 steps to Crystal City, Missouri, on March 10, 2002, traveling a total of 1,231 miles in a bit more than a year and a half.  "Where's George?" has grown into something of a cottage industry.  On the site you can buy t-shirts, bumper stickers, even soap with a dollar bill inside.  It has also given rise to a host of similar sites devoted to other currencies:  Canadian dollars, Japanese Yen, British Pounds, and European Union Euros. 

Money tracking on the Internet also has a more scientific side.  The launch of the Euro on January 1, 2002, has provided an opportunity to use Internet tracking to study European integration.  Each 1 € coin has the same design on the "heads" side, regardless of which country it was minted in, but the "tails" side is country-specific.  Germany, for example, has the German eagle on its one Euro coins, while France has a stylized tree set in a hexagon, and Italy has the famous drawing by Leonardo da Vinci illustrating his view of the ideal proportions of the human body.  Researchers in at least two European universities have been tracking the diffusion of the coins to test alternative hypotheses about how long it will take for the coins to distribute themselves throughout the Euro zone.  Such a study, depending on the participation of thousands of volunteers and their ability to easily report findings through the web, would not have been feasible without the Internet.

Links:
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Where's George?  The mother of all money tracking sites.  This project involves submitting the serial numbers of U.S. dollar bills along with the zip code indicating where it was obtained.  The site tracks the travels of the various bills that have been registered.  If the bill that an individual submits has been registered before, the submitter gets a list showing where it has been before.  To date, the site has nearly 1.3 million users.

Where's Willy?  A Canadian counterpart to Where's George.

The Freiberg Euro-Coin Projects.  Two scientific studies on the mixing of Euros by Prof. Dietrich Stoyan at the Institute of Stochastics of the Technical University Bergakademie Freiberg, Germany. 

Eurodiffusion (English Summary).  A parallel study conducted by a group at the University of Amsterdam (Netherlands), including Misja Nuyens, a doctoral candidate in probability theory.  The project's web site is hosted by the Wiskundig Genootschap, the Dutch Mathematical Society. 

Eurodif 2002.  A study of the diffusion of the Euro based at the Polytechnical University of Madrid, Spain.  The graphics page generates charts in response to specific queries.

Otto Pohl, "Statisticians Count Euros and Find More than Money," The New York Times (July 2, 2002).

Eurobilltracker.  A European site similar to the U.S. site, "Where's George?"

Doshtracker.  A money tracking site for the U.K.

Where's the Money? The site's name is in English, but everything else is in Dutch.

Osatsu.net.  A Japanese bill tracking site.  (English versions of most pages are available.)

BookCrossing.com.  Track books instead of bills.  First you have to register the book, then you "release" it--i.e., give it away, lose it, etc.

Phototag.  This is a photography project.  Every so often, its organizers label a number of disposable cameras and "release them into the wild" with instructions for those who find them to take one picture and pass the camera on.  Postage and a return address are included on the camera so that whoever takes the last picture can mail it back to the organizers.  The site reports on the cameras' wanderings and displays the pictures.