I have already seen a number of people posting the "where I was 10 years ago on 9/11" thing so I thought I'd add my voice in since mine may be different from the majority of Americans.
I was in South Korea on my 2nd month of a 13 month stay teaching English in a girl's high school. I had just moved in with a Korean host family and was sitting in the family room while they watched a Korean prime time television show (it was 10pm-ish on 9/11) and I was on their computer checking email and probably scanning photos to post online.
A crawl showed up on the bottom of the TV screen and my host mother said that it sounded like a plane had hit one of the buildings in New York. So I tried to get on a news website but none of them were loading. I couldn't get on to any new sites. Finally my dad came online (he and I chatted multiple times a day via AIM) and told me that a commuter plane or something had hit one of the towers. Then he essentially narrated the second tower being hit. Nothing was really known but my dad said they thought maybe it was a terrorist attack. Finally I had to go to sleep.
In the morning, there was nothing about the attack on the English-language paper I got every morning (I guess they went to press earlier in the night). However, I was able to finally get onto news sites and was shocked to find out that not only had the towers collapsed, but the pentagon had been hit and a fourth plane had gone down and they were calling it a terrorist attack by Al Qaeda.
It was pretty strange. Here was I was halfway across the world while all kinds of crazy stuff was happening. My friends and family in NYC were all fine. My brother in Ithaca, NY was fine. My mom in Chicago was convinced the Sears Tower was next and the 30-something of us on Fulbright Teaching Assistantship grants in South Korea were not quite sure what to do. We were told that we were safe where we were (spread out across the country all in different cities) and to let them know if we couldn't get in touch with any family. I was able to communicate via phone or email or IM and spoke with my mom to assure her I was fine.
I think I had a much more detached experience than anyone in the US. I still had to teach 5 classes of 40 students each that day. I'm sure I could have gotten out of it if I wanted to but I didn't feel as overwhelmed or worried as somebody in the US may have. So we just kind of continued on. I paid a lot more attention to the news than I may have otherwise and certainly spoke with my family a lot.
So that's my "where were you on September 11th" story