I remember driving to work that morning when the 1st plane hit. I was listening to a talk radio morning show, and one of the hosts interrupted and said "Oh my God! An airliner just hit the world trade center!" I remember being outraged, thinking "How the heck can some pilot screw up that bad with navigation, and cause that bad of an accident! He shoulda been no where near that building!" It never dawned on me it was an attack. They basically said they'd give us more news as it was available, and then went on with the program. Later on in the show, she broke in again, and said "Another plane just hit the other WTC building!!!"
Then I knew it was an attack! When I got to work, my building has a lot of monitors in the halls (my company does a lot of work with NASA, and there the monitors usually have some spaceflight live camera stuff on them or reports from a mission.) People were crowded around the monitors, which showed the first building collapse, and the second building on fire (and later also collapse). I remember looking around at all the other people, and seeing both men and women crying. Later the building security staff locked all the doors, and we couldn't leave. Our managers were running around frantically, taking attendance, etc.
Later on in the day, in our cafeteria, I looked out the window and saw what looked like the presidential helicopter, with two F15's flying in circles around it as it went- they were fairly close to the building when they went by- clearly close enough to ID the types of planes. I saw an older women looking at them, and her face went white as a ghost. I told her "It's OK, they're US military planes", and she thanked me. I know that sounds lame, but I could tell she was afraid we were under attack there!
Anyways.. I also remember how clear blue the sky was- not a cloud in sight. And, also no planes in the sky for a few days. Crazy stuff.