Found two funny sites that let you see the route that your bills have been doing across USA (dollars) or Europe (Euros).
It's a kind of a distributed human project, you insert the bill's serial number and it shows the places where it has been reported by other people before.
Take a look

http://www.eurobilltracker.com/
(For Euros, needs registration)

http://www.wheresgeorge.com/
(For Dollars)