My girlfriend at the time was a 911 call taker for the New York Fire Department. Not knowing exactly where she was based, I spent most of the day trying to get in touch with her cell phone -- but her cell phone tower was on top of one of the Towers.
I remember turning on the TV not long after I got home, just for background noise as I got ready for bed. An announcer was saying that one of the towers was burning and another had just been hit by a plane, and I remember thinking what a horrible coincidence that was -- some TV news plane must have been circling around the fire when it hit the other Tower.
My fire department got placed on standby, as did every other department in the country, so I spent most of the day at the station, staring at the TV.