Unfortunately not ALL of the museums are free any longer. The NEWSEUM is actually quite expensive. The Smithsonian is still free but if you wish to visit the Udvar Hazy Air And Space Museum out near Dulles Airport there is a rather hefty parking fee. I also think their may be a fee at the Holocaust Museum.
I do agree with Johnie though about visiting the DC area. There is SO MUCH history here.
Oops good catch! I meant to say all of the Smithsonian museums are free
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