I rarely have cash and I don't always remember to mark them, but for the first time ever, one of the bills I marked (am I admitting to an illegal act*?) and entered into WheresGeorge.com actually got re-entered. I'm bummed to see it traveled at a speed of 0.14 miles per day over the past year and a half (I could have walked there faster), but it really makes me wish I had more money to mark and release.
WheresGeorge.com is a fun little site. You create an account, and then you enter in the serial number of all your bills. Then you write "www.wheresgeorge.com" on them. And then you spend them. Other people go to the site, enter in the serial numbers and you can start to see where your money goes.
I have entered 31 bills worth $971, surprised no others have shown up.
(*No. It's only illegal if it makes the bill unusable or is done in attempt to fraudlently gain from it.)