Smithsonian Museum of American History
This museum hosts two exhibits from the National Numismatic Collection:
The Value of Money (http://americanhistory.si.edu/exhibitions/value-money)
Stories on Money (http://americanhistory.si.edu/exhibitions/stories-money)
Must-see for coin enthusiasts
If you're a coin collector and happen to be living in or visiting D.C., the money gallery at the Smithsonian American History Museum is as must-see. The collection includes, among others, the 1974 aluminum Lincoln cent, as well as multiple St. Gaudens gold coins.