www.MoneyTracker.com.au is a great site that lets you track the location of your bank notes after you spend them.
Basically you go to the money tracker web site, and enter the serial number of any bank notes (cash) you have. Then you just spend them like you normally do. Eventually, someone else will get the note, enter it into money tracker which then notifies you of the new location of your bank note.
This fascinating idea was inspired by the US website http://wheresgeorge.com/ which lets you track U.S. bank notes.
Now all we need is some sort of machine that can help us get our money back after we spend it 🙂