I was sitting in a college class, though I had no idea what had occured till midday ( I had a tendency of going back to sleep after my 8 am class, in part due to midweek Breakfast shows - though I never missed a class no matter how late I stayed up). When I woke up from the nap, I heard about all the events and spent the rest of the day wondering if any other horror was planned.
The strangest part of that morning was a lesson my teacher was giving about how he made the elevators more efficient in a high-rise building while on a consulting gig. I remember thinking about how dangerous a tower could be in the event of a fire or monstrous earthquake, just a few minutes prior to the events.
A girl from my high school died due to being stuck on a floor above the impact zone. Seems like everyone knows someone that was affected.
I have also heard a lot of remarkable stories about people calling in sick or coming into work late on the day of the attacks.