I was at work. I hadn't heard anything until I went to my first meeting, and someone came in late saying the second tower collapsed. I had to ask what tower - and he told me and another co-worker what had been happening all morning. I couldn't believe our account manager didn't say anything to people.
My best friend worked in Manhattan, but I couldn't recall if her company was in the World Trade Center or not. I finally got a hold of her - she was fine. She worked in an office building across the Hudson - she saw the whole thing first hand, and told me how everyone had to walk home that day due to the whole city being shut down.
Will never forget that day.