I was serving (mandatory) in military in my home european country, guys at our guard platoon desk were listening radio talking about some crashing plane, I just went outside the barracks in my spare time to netcafe to do some surfing and just noticed in news sites talk about some plane crash.
When I came back I've noticed we have suddenly CNN channel instead of our regular local TV station (they switched from regular programming to airing CNN live) with South American telenovelas (the guys watched it for pretty babes, I stayed usually with clerks around computers in captain's office playing games or chitchatting).
Then in the evening came barracks chief (don't remember position, I guess Mayor) telling us we are on high alert (though we didn't understand what have something in US to do with us in small european country, not even in NATO back then), we had to double guarding strategic objects (we were nearby NPP and also ammunition storage in forest).
First night after attacks I've spent on night shift next to fax to receive updates, luckily nothing interesting came and WW3 occured and we just continued with doubled guards for some time, which was actually better for the regular soldiers (not for me - office rat doing clerk work helping professional soldiers in office with paperwork and I had to go actually guarding 24hr shifts instead of sitting at computer and sleeping in our own more comfy clerk room), at least you had someone to talk to while doing your 2 hours round in cold forest. In the end they didn't prolong mandatory conscription (many were afraid they are gonna do it because of this) and we finished on time at the end of next April.
I doubt if I were not serving in military during 9/11 I would remember this day in such detail as European not affected by it (other then annoying heightened airport security later).