The towers falling is one of those memories that stay with you forever. Not just a memory, but you are able to remember where you were and what you were doing. Other ones that come to mind are:
The space shuttle Challenger exploding with Christa McAuliffe and six other brave souls.
Ronald Reagan being shot.
The bombing of the federal building in Omaha.
And to a lesser extent, watching the police chase O.J. Simpson and Al Cowlings in the white bronco.