Well, look who's all Web 2.0-y now? Being serious, this is a very, very well-designed site. Whoever did NASA's work for them deserves much credit.
It's all built on Prototype+Scriptaculous, and is very well executed -- check out the expanding "Blinds"-style menus at the top. No Flash there. (In fact, if you ask me, those menus on NASA's site are a damn site quicker that I've ever seen Flash do.)