I was on my way into the city to go to Chelsea Piers (which they used as a blood donation/sick bay that day). I turned around halfway there when we heard "rumors" that a plane had crashed into the first tower.
My brother lived in TriBeCa at the time, though, and he and his roommates had to evacuate the city. My brother wasn't allowed to go back into his appartment to get his dog! His female roommate had to sweet-talk her way in just to get the poor dog.
All the stories of "I was supposed to be there, but I needed to stop here..." amaze me. I know tons of people who were supposed to have meetings there that morning or that work there and they stopped to get coffee or were running late that morning. I don't personally know anyone that died in the WTC, but I know people whose parents passed away or who had friends. It's all very devastating and surreal.