I came across this about a year ago, and I've played casually with it since then. There is a website out there called Where's George that is something of a Where's Waldo for anyone who ever comes across American currency.
The trick of the game? Enter as many bills as you can get your grubby hands on, spend said bills and see how many get entered again, how far they travel, and so forth. There are even places online where you can buy stamps to mark bills before sending them back out into the population (with bright red ink to capture anyone's attention so they can start doing the same) to circulate. No, it isn't considered defacement of currency as the bills are not damaged and are still able to be spent.
Go check it out and join the fun!