Did you know that there is a website that tracks specific dollar bills to see where they've been spent and what condition they're in. I had no idea such a thing existed until I got a dollar bill at the library that looked like this:

I went to Where'sGeorge.com and so far my dollar was first registered in Maryland and then I put in my tracking information. There is a list called George's Top 10 which shows the bills that have been tracked the most. One bill has been in Alaska, Canada and Hawaii -- it's more well traveled then I am! You can even continue to track your bill after you've spent it by registering on the website. I think I'll spend my dollar on coffee -- so look for it around Rockville Centre, and if you find it be sure to enter it on the website!
If you are going abroard be sure to check the links sections of Where'sGeorge.com and you can track euros, pounds and even rubles.