Some sites encourage marking before spending, others don't. This usually depends on the laws of the country issuing the currency.
Where's George is the first site to start this kind of activity. It was created by Hank Eskin. The site has inspired several other ones, see the list of external links.
Eurobilltracker was started on January 1st 2002 (and because of this the German site "Wo ist mein Geld?" stopped tracking).