More than 250 historically significant aircraft and spacecraft of the world now reside in the Steven Udvar-Hazy Center of the Smithsonian Air and Space Museum. One of my personal favorites is the B-29 Superfortress, the Enola Gay. Simply put, she was the beginning to the end of the War in the Pacific Theater as she carried the first atomic bomb to be used in the history of warfare. If you are looking to visit the center, I would highly recommend going with a whole day's worth of time ahead of you, especially if you like either airplanes, war history, or anything to do with space! (I fit in all three of those categories!)
Walking these spacious halls, it is amazing to see just how far the science of flight has progressed from the Wright Brothers' Flyer to the colossal jets like the supersonic Concorde and the Discovery shuttle.