I have NOT yet made it to Dulles, as it wasn't open last time I was down, but I'll add one that is great if you are REALLY into airplanes, and if you can get out to Silver Hill MD - The Garber Restoration Facility - I think it's 2 tours/day, limited numbers, and NOT every day. It's where they restore the airplanes, and where they (at least used to) store the planes that are not on display. You're only allowed in a couple of the hangers, as the rest are closed to the public

Ooops - nevermind - no longer open to the public, as most of it has been oved out to Dulles

http://www.nasm.si.edu/garber/