I was in New York that day, on a film set.
I drove to work early that morning and as usual, parked at the Port Authority terminal on 42nd St, then travelled by subway and a production courtesy van to the studio in Queens, just a few miles away.
We were on set and had already been shooting for nearly an hour when one of the crew members relayed the news that one of the towers was hit by a plane...there weren't many details at that point, and the filming continued. We all assumed it was a small plane, and an accident. I had a bit of a break very shortly thereafter and joined a couple of the other cast members and some of the crew guys on the roof of the building since it offered a clear view of the towers. As we watched the plume of smoke rise, there was a flash, then more smoke. We didn't know until we went back downstairs to the set that from our vantage point on the roof we had just witnessed the second plane hitting the other tower. We continued working as further news trickled in and shortly thereafter, our director, Sidney Lumet, gathered cast and crew together and said that anyone who felt that they had to leave (or was able to leave) was free to do so, but that the shoot would continue as long as possible with those that chose to remain. I stayed put...my car was in Manhatten, only a few miles away, and really couldn't go anywhere...the city was essentially locked down, and it was pretty much impossible to get in or out with all bridges & tunnels shut down.
So the shoot continued with only minor changes to the day's schedule since the star of the show, Alan Arkin, had also opted to stay.
We actually worked until 7pm, the usual wrap time, and an assistant director who drove to work from New Jersey (where I live) and who was parked in the studio lot, kindly offered me a lift home since there was no way to get to where my car was in the city. A drive that under normal circumstances should take no more than 70 minutes stretched to nearly 5 hours since we had to take many detours to the only lane on the only open bridge out. As we drove through the lower part of Brooklyn (at that point, relatively close to the disaster site) we experienced the heavy smoke in the air with the smell of burning jet fuel, debris, and lord knows what else.