Follow your money

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)

Hilarious and fascinating to read one of their sample bill travel reports! I'll look out for marked bills now and maybe mark and start one.

How much info do they want from you in their registration process? That's the only thing that makes me wary. Did you get any spam after signing up?

Actually, I didn't get register myself on the site. :P