I have always been interested in the design of paper money. The craftsmanship and artistry that goes into banknote design is admirable, and the anti-counterfeiting features are interesting to learn about. If you know anything about paper money around the world, you will be aware that U.S. currency is quite drab and dated compared to some. This short video traces the history of the American dollar bill, and shows that although it has always been something of a design trainwreck, it has shown occasional bursts of beauty (and a surprising amount of color) at times. The backs of the bills hold particular interest for me, because it seems like there is where the most interesting stuff happens, design-wise.
Via kottke.org, an interesting look at the history of U.S. postage stamps, as well as how they are designed and produced.