Certainly watching the 2nd Plane hit the Towers on 9/11. My wife called about the first plane, but I wrote it off to a small plane, but then when I surfed the CNN site and saw it was a passenger jet, and then went into the Cantina at work to watch the second one hit... The rest of the day was pretty well shot, work-wise, and most everyone I know was glued to the internet or the TV watching the story unfold.
After that, the Columbia breakup (home watching the expected re-entry), the Challenger explosion (standing in line in the chow hall at Great Lakes Naval Station after boot camp)... But 9/11, unsurprisingly, sticks out the most in my mind.
