Went to the International Spy Museum. This is very good and highly recommended on a visit to the US capital. There is a special exhibit on terrorism on at the moment. Americans are pretty used to terrorism. The exhibit details the anarchist bombings of the 1880s, the Red Scare of 1919-20 and the Sedition Act of 1918 which made it illegal to "utter, print, write, or publish any disloyal, profane, scurrilous, or abusive language about the form of government of the United States, or the Constitution of the United States, or the military or naval forces of the United States" [Penalty: $10,000 and/or 10 years imprisonment]. (Fear not, American readers, the US Supreme Court ruled that this did not violate the First Amendment. And it was repealed in 1920.) There's the revolver that President McKinley was shot with. There are Ku Klux Klan (the oldest terrorist organisation in the US!) robes on display. (Did you know that President Harding was sworn in as a Klansman in a ceremony at the White House?). There's a display devoted to the terrorist organisations of the 1970s like the Weathermen (and video of interviews with some of their members). And they even have a chunk of the World Trade Centre!
In the permanent exhibits you can find bugs and eavesdropping devices of all kinds (and listen in on the conversations of your fellow tourists!), they have spy cameras, a WWII Enigma machine, interviews with various CIA, FBI and KGB spies, pictures of famous spies like Mata hari and Diana Rigg and even James Bond's Aston Martin!