When I was paying for brunch on Sunday, I noticed that one of my $1 bills had a pink stamp on it. It said:
Currency Tracking Study
Reporting Sightings To
I visited wheresgeorge.com to enter the serial number of the dollar bill and was presented with all the entries written by others who had received the same dollar bill and had gone to the website. I was expecting a slew of posts, so was disappointed to find that there was only a single entry made before mine that was written nearly two years ago. I hope more people will record sightings in the future because I've registered to receive notification of any updates.
BTW, you can do this for any U.S. or Canadian paper money in your possession. It's all explained on the website.
This reminds me of a movie I saw called Twenty Bucks starring Brendan Fraser. The movie tracks a $20 bill, and tells a story about how each of its owners receives it, uses it and then passes it onto the next person. It's an interesting concept and I enjoyed watching it. I recommend you rent it some time even though I haven't described it properly.