USAF Museum (technically the National Museum of the United States Air Force) in Dayton, Ohio.
Largest and oldest military aviation museum in the world. Three main hangars, a fourth that'll be built soon, and two additional hangars on the active base. They have pretty much every spectacular aircraft ever built - including the B-29 Bockscar (dropped the bomb on Nagasaki in WWII), B-17 Memphis Belle, XB-70 Valkyrie, the only B-2 on display anywhere in the world, the original Air Force One that carried JFK's body back from Dallas, etc.
If you're an aviation nut, it's a must-visit!
Of you can't visit, they have a
spectacular virtual tour:
http://www.nmusafvirtualtour.com/full/tour-std.html
Some photos I've taken during my many visits there:
http://amileofrunway.blogspot.com/se.../USAF%20Museum