RIP Ayrton. I don't remember the day, as I was only 5 and started following F1 from 1997, but everything about that weekend was appalling. Barrichello's crash on Friday was a very lucky escape, and he was lucky not to be worse off after the way the marshals just dropped the car after flipping it back over. Ratzenberger's crash, Senna's crash, Lamy and Lehto's crash, that tyre flying down the pit lane.. it was a disaster from the start. I hope we never have a weekend like that in F1 ever again, and I'm pretty sure we won't. Safety has come a long way since 1994 and very rarely have we had a "serious accident".
Only ones I can think of are:
13 cars - Spa 1998, Fisichella/Nakano in the same race was pretty ugly.
Schumacher - Britain 1999.
Burti - Spa 2002?
Alonso + Webber - Brazil 2003
McNish - Japan 2003
Thankfully nobody was killed in those, but bad accidents are getting rarer.
You could add Kovalainen in Spain last year to that, I was seriously concerned when he crashed. Feel free to add more, those are ones that instantly spring to mind.