I came across this website when G2 paid me a dollar he owed me (under protest of course). The doller bill it was stamped with www.wheresgeorge.com, so being the nosy one that I am, I checked it out.
It turns out that it is a dollar bill tracking site. All you need to do is key in the denomination, series, and serial number of any US dollar bill, and if others have registered it, you get a report of where it has been. The bill that G2 gave me was in Denver in 2004. You can also have the site email you if your bill is registered again. I thought that this would be a fun activity if you have younger kids (well I guess for us big kids too).
They also have a site that caters to our Canadian neighbors. You can find this one at www.whereswilly.com. It looks like it does the same as the WheresGeorge site.