I was on the first floor of the sign manufacturing company that I ran back then, in Swindon, in the SW of England.
 
My co-director always had the TV on and we stood around the huge layout bench watching in disbelief as the appalling events unfolded.
 
I still remember that a sales guy we had working for us acted very strangely, bustling around almost maniacally with his work..looking back he was perhaps trying to pull the day back to 'normal' I guess.

I was a security officer, off duty that day. I was still in bed when I got a phone call from my boss telling me to show up asap. 
 
I had no idea whatsoever what was going on until I actually got to work - the people in the office where I was keeping watch brought me up to speed as to what was happening during their breaks.
 
At that point, no one in there had seen any image, all the info we had came in from conference calls. It wasn't until after I left work that I saw the footage. My roommates and I all starring at the screen, in absolute disbelief. It felt like the end of the world.
 
My younger brother who had just arrived in town and was hoping to find work was hired as a security officer on the spot that same evening IIRC - or the next day, but on the spot whenever it was that he showed up. 

I was working in the EMS evaluator's buggy that day for DCFEMS. I heard about the first plane on the news station on the AM radio. Stopped at 8 Engine and saw the second plane hit on TV. Left the firehouse and got about two-three blocks away on East Capitol when they hit the box for a report of a plane into the White House. Started to respond on that, and while coming around the Capitol they struck another box for mutual aid to the Pentagon. As I hit Pennsylvania Ave I could see the smoke from the Pentagon. A few more blocks up Pennsylvania Avenue they struck a third box for a report of a car bomb explosion at the State Department. I called my wife, told her not to worry, and not to expect me home anytime soon.
 
The car bomb turned out to be false. We thought we might relieve the guys over at the Pentagon, but they kept us in an underground garage as a quick response strike team should something else happen. The folks at Shanksville responded to the plane that was heading for the White House, my original call. I've been to Shanksville several times. I always say a prayer of "Thanks" to those folks.
 
Long day. We lost a lot of really good people.

I was working second shift on the west coast, so I was expecting to sleep in.  I'd had a terrible night trying to get to sleep and I woke up pretty early without an alarm.  It was like I felt something going on.  Turned on the TV and it took me a moment to figure out what was going.  Then the first tower fell. 

Working, at the drape and blind company in Terrell, TX. The seamstresses always had a radio on (usually to some type of music I didn't like) and they interupted their playlist for the announcement of the first strike.
Like many folks, I assumed it was an accident, since planes have hit buildings many times over the years.
The second strike proved that idea wrong.
The ladies were weeping. I was angry.
 
When I got home from work last night, the room mate was watching "120 Minutes that Changed the World" on History.
A scene near the end struck me; hundreds of firefighters carrying their gear, maybe a hundred pounds each, walking many blocks toward the towers, and knowing that when they got there they would have to climb perhaps 50, 60, 70 stories to get to the fire.
And they still did it.
 

I was checking out from a hotel in central Finland and noticed a group of people gathered around the TV in the lobby.
That was at about 10 am north European time (GMT +2), so it had all happened already, both planes etc.
I quite couldn't believe I really got it right. I went to the car where my colleague was waiting, and told him. He called his wife immediately to make sure she's watching. 

I remember for weeks afterwards, the absolute first thought on my mind when I woke up every morning was "I can't believe they actually did that." And then I would lie there trying to imagine myself in that cockpit intentionally flying into a skyscraper and just thinking it was impossible that someone would actually bring themselves to do it, then reminding myself that they actually did. I think it was the first time I realized how fanatical and far removed from reality these barbarian refugees from the 7th-century really are. 
 
Every year around this time there'll be a surge in 9/11 footage videos posted online and I'll always end up amazed at how much I haven't seen. I watched one a couple of nights ago which was 30 minutes of some guy with a video camera wandering around Ground Zero right after the first tower fell when everything and everyone is covered in a thick layer of dust and people are wandering around confused. He gets right into the north tower and wanders around for a bit as cops and firefighters run around frantically. Then he goes back out and films the flaming ruins of the south tower and talks to some of the people around the scene. Pretty soon of course the north tower starts collapsing right above him and what amazed me was, as everyone around him runs for their lives, he not only stays where he is but actually stops to wipe his lens - as it's collapsing - to make sure he gets good footage. It's amazing how people's perception of danger changes when they're behind a camera. 
 
Of course nothing touches the Naudet brothers' documentary. It's absolutely horrifying footage but probably one of the most historically significant accounts of that day. Your heart rate will double while watching it. 

I was in my truck, in front of the plumbing shop, waiting for my boss to arrive.
My wife called and told me the World Trade Center just fell down.
I told her,"the World Trade center doesn't just fall down". There had to be some mistake.
Tim, my boss, showed up, we went inside and turned on his 5" black and white TV.
There was only one tower. What?
Moments later, tower 2 went down.
I don't know how long we sat there, trying to absorb what we had just seen.
The rest of the day is gone, lost in a haze.
That huge image on that tiny screen is seared into my mind forever.

I was in Saudi Arabia on an oil camp. There were probably around 20 different nationalities, evey one equally shocked and stunned.