We had a friend who was working at the Pentagon at the time. He was okay.
A friend of mine, whose DH had worked in the Towers, knew a lot of people who were caught up in it. She was very, very shaken up by it all - I spoke to her on the phone at the time, and she had no idea what had happened to her DH's former colleagues. Luckily they got out, but she didn't know that for several days.
I can remember watching it on television after DH rang me from work to tell me what was going on. The whole scene was appalling anyway, and then when the first tower came down, it went from horror to apocalypse.
The world - certainly the western world - changed that day, not just America. The general feeling that civilisation was an upward trajectory towards a decent life for everyone took the hell of knock as we realised that there were people out there who hated other people so much that they were prepared to kill 100s of civilians out of the blue, and who despised their own lives so much, compared to whatever promised paradise they were expecting, that they were prepared to die in blazing fireballs. It felt as if no sooner had we managed to shunt the Soviet Union, with its millions of dead, off the stage of history, than something new arrived for us to worry about, and fear.