I was in High School and remember being on my way home; I got a text from my dad saying "turn on the TV when you get in"; I asked why? and he just replied "chaos in America"; a the time I think I replied with "Whats new?".
Anyway; I had just got a WAP enabled mobile (those were the days!) so I could get basic teletext style news and was just staggered; I think that was just as the second plane has hit the WTC and I just though; what kind of ATC failure is that?
I got in and my mum already had Sky News on and I think that was just as the second tower collapsed; and there seemed to be a lot of confusion about how many planes had been hijacked; car bombings in Washington D.C, etc - it was chaos.
I remember as well that buildings in London had started being evacuated at that point and there was genuine fear we were going to see something similar here (Although I now know that was unlikely given our stronger security on domestic flights)
I couldn't believe what I was seeing and was convinced World War 3 had just started but I remember the wall to wall coverage going on for days after that; its certainly going to be one of those "where were you" moments in my lifetime. I certainly will never forget the sight of people jumping from the two towers.
I iniitally bought into some of the conspiracy theories (I was only about 14-15) but not now; its pretty clear that as a collective the US intelligence community had enough information to potentially disrupt the attack but was run as competing agencies (CIA, FBI, NSA, etc) rather than a more collegiate effective group of agencies; it was ultimately the biggest failure of US intelligence since Pearl Harbor and perhaps up until their assessment of the Taliban and Afghan Army's capabilities back in July...
I understand why 9/11 still talked about and remembered so much; it was unprecedented and thankfully never repeated on the same scale but its also nuts to think that we only just came out of Afghanistan a couple of weeks ago. Its had such a massive lingering impact on the world; the War on Terror, Arab Spring, etc; in some respect its had a bigger impact than the 2008 financial crash did.
A few folk mentioning 7/7 as well; that's also fair seared into my brain; I was at uni but was studying at home; turned on the TV just as the first "incident" had happened and London Underground, etc were reporting it as a power surge; I just knew immediately it was a terror attack and I think it was about an hour later that the Met, etc officially confirmed that.
7/7 probably caused more panic/fear in the UK; it was the first suicide attack on UK soil.
I was going in and out of Edinburgh for Uni every day at that point and I certainly became a bit more vigilant and cautious even though I thought it pretty unlikely here; that said even now when I'm in London for work I do tend to keep my eyes peeled, etc.
As a nation we're certainly more on edge that we were before 9/11 and 7/7.
Back to 9/11 - there's a documentary (6 parter) on National Geographic this week (and last I think) called 9/11: One Day in America - some rehashing of the usual but I hadn't seen some of the footage from the WTC before; the noise of people jumping and hitting the roof of one of the buildings below as the Firefighters are inside is just awful.