Yeah, I know I've mentioned it before, but I track my currency at WheresGeorge.
Yes, it's the geekiest thing I do(I hope!). I joined about a year and a half ago, and just yesterday I received the 1000th hit on one of my bills. I've registered 15,410 bills with a combined value of $70,708. There are a little over 50,000 people in the USA actively tracking their currency at WG. I'm in the top 100 Texas trackers. woohoo!
See? I told you it was geeky.
There are some WGers who have registered more than a million individual bills so if you think _I_ am obsessive, well, I'm chump change. Srsly. It's interesting to see where your money goes, and in some cases, how quickly. If I travel, I always try to get a couple straps of ones from the bank and register them to disperse along my path. Yes, this means spending them-- not just throwing them out the window of the hot air balloon.