While I was only 16 at the time, I had an internship in New York during the summer of 2006. One day I happened to ask my manager if he was in NYC on September 11. He proceeded to hold all his phone calls for about half an hour (a lifetime on a trading desk) to tell me his story. He was working at Barclays at the time, and his building was just a block or two from the World Trade Center complex. They felt their entire building shake and assumed a bomb had gone off. As he is a corporate bond trader, he worked very closely with folks at Cantor Fitzgerald (you can look up the story of that company if you don't know about it already). He said that the most horrifying moment of his life was watching all the direct phone lines to people at Cantor go dark, one by one.
They had to evacuate the building, and he was on one of the very last (perhaps the last) New Jersey Transit train to leave Manhattan. Before he boarded the train, he saw the towers on fire. By the time he reached New Jersey and looked back toward lower Manhattan, both towers were gone. He then spent the next 9 months or so attending funerals for all the friends he had lost. Pretty heavy stuff. Let's hope such a tragedy never happens again.