I stumbled across a site called Where's George http://www.wheresgeorge.com and I have to admit, I'm intrigued. You enter the serial numbers of your paper currency into their database, which searches for other entries of the same bill. This lets you track where your bills came from, and where they've gone. Seems simple enough, but it looks like it can become quite a geek addiction (right up there with MacRumors and WoW). Pretty interesting forums on the site as well. Anyone have any experience with this? I'm thinking of diving in.