wow, were you there when it happened or when they were cleaning it up?
I should clarify. I believe that I saw "the plane" that hit the Pentagon, not that I saw a plane "hit" the Pentagon.
I was working at the Naval Research Lab at the time. NRL is close to national airport, and so you see planes flying down the Potomac all the time. It is also only a few miles from the Pentagon. On the morning of 09/11, I was walking from the cafeteria (had a late brekafast) to the chemistry building, which is on the other side of the lab (or at least it was at the time). I liked walking down by the river, so that was the path I took on that day. While walking I noticed a plane flying low and relatively near the river, and banking hard right. Did not think much of it at the time, as plane's often bank like that to come into National. So it was not an unusual sight. Only a couple minutes later, there was a loud BANG and smoke poured up from the area past National Airport. By the time I got back to my building, reports were coming in that a plane hit the pentagon.
I did observe the damage to the Pentagon while driving by it later than week, as it was clearly visible from the highway. And FWIW - my uncle worked in the side of the pentagon that was struck, and lost many of his staff. He got "lucky" and was visiting a contractor in New Jersey that day.