Dolla Dolla Bill, Y'all
Inspired by Matthew Yglesias pointing out that Jay-Z is conducting business with a stack of 500-euro notes in this video, I did some searching around the web for info on U.S. currency. Wikipedia, of course, has plenty of info on things like the two-dollar bill and large-denomination bills, but if you're not bothered by extensive editorializing, this guy will teach you all about the six different kinds of U.S. currency (i.e., Federal Reserve Notes, Silver Certificates, etc.) as well as giving you his opinions on just about every aspect of monetary policy. If you're interested in odd foreign currency, Wikipedia will show you examples of hyper-inflated currency, like a 500-billion-dinar Yugoslav banknote from 1993.