I was on my way to my first day of work. I didn't have an apartment in Manhattan yet, so I was coming from my brother's place upstate. The train was just coming around a bend by the Meadowlands when people noticed the WTC on fire. When I got through the tunnel from Hoboken, we learned of the second tower being hit. It was clearly not an accident given the clear weather, but this just confirmed that.
After making my way to my office in Midtown, it was obvious people didn't know quite what to do. Listen for new information, stay in the building, try to go home. I took my then-girlfriend to meet her father who'd come in from Brooklyn for her, and then left for downtown. I still had a military ID, so I was able to get through the checkpoints and make it there.
There was a staging area at Stuyvesant High School where military personnel - active, reserve and National Guard from all services - caught in the city were assembling. There was also a quick sojourn to a National Guard armory in Midtown to pick up equipment which involved myself and two ladies speeding north in an FBI SUV. After a few hours of waiting while they tried to figure out what to do with us and while the active personnel used Bloomberg's facilities and its separate network to contact their commands to let them know where they were, and a very officious prick from the New York Army National Guard tried to assert control despite being severely outranked, a large group of us just said "F' it" and went straight to Ground Zero, where we found that the FDNY and NYPD didn't care what uniform you were wearing, just whether you could help.
After that it was hell. Recovering bodies and body parts, hoping in vain to find someone alive, having to unmask and breathe in whatever was in the air because smell was the only way to find anything. I had grabbed a Halligan tool off a destroyed fire truck and lugged it around for the next several days; I still have it. Called work the next morning - was surprised to finally get cell reception - and let them know I wouldn't be in for a while.
The people I was with were an eclectic group. A full colonel from Army Civil Affairs who'd jumped in his car and driven all the way in through various roadblocks from Connecticut, a lieutenant colonel from the New Jersey Air National Guard who was a trauma surgeon, a couple of MPs, a Navy corpsman, two Army Reservists with the sadly ironic MOS of graves registration specialist, etc. Good people.