I remember where I was during nearly every major accident in the last 15 years, but since the two most discussed on this thread are AA587 and 11SEP, I'll share those.
Had just landed in Jacksonville, FL. Preparing the cabin to begin boarding, when another flight attendant came down the jetbridge, and said "your not gonna believe what's happening in New York." I brushed her off at first, until she said....two airplanes just flew into the WTC towers within 15 minutes of one another. I went up and began talking to one of the agents, and he explained that one was an American 767. Shocked, I came back to the aircraft, and we began boarding. The other two flight attendants were in the forward galley when the captain called me in the back and said there was a nationwide ground stop, and that another plane had just flown into the Pentagon. He was going down to call S.O.C., so I got to then explain to the passengers via the P.A., that "due to the circumstances in New York, air traffic across the country had been halted. Although we have very little information, everyone will need to take their carryons and return to the gate area." Of course, things just got progressively worse from there (I got to spend 4 lovely nights in Jacksonville, FL).
AA587 - Just completed boarding in Atlanta, and were getting ready to depart for Philadelphia. Stepped out onto the jetbridge to pick up my manifest, and saw the agent coming down to the aircraft. About 30 yards away, her waist phone rang, and she picked it up. She just stopped and stared at me. When she got to me she had this look on her face like she was about to burst into tears. She said "an American Airlines 767 just crashed in Queens, NY". My only response was "?!?!?!?!?!?! huh?" With that we, shut the main cabin door, and I told pilots, and another flight attendant who was standing in the forward galley. Since it was departure time, we had no choice but to press on. It was only when we arrived in Philadelphia, and headed for the hotel that we saw on the news in the airport that it was an A300, and that it didn't immediately appear terrorism was suspect.