1 - The Wall Street Crash of 1929
2 - Black Monday, October 1987
3 - Tech Stock Bubble Bursts 2000-2002
4 - Subprime Debt Crisis 2008-2009
With all the talk of a forthcoming US fiscal cliff this might be relevant more immediately than expected with the DJIA shedding some ~430 points (or ~3.5%) over the past two trading sessions.
The markets have returned to high alert mode stateside, dropping to their lowest level since July, when all the talk was of Greece (the Greek austerity measures and government debt pressures are still rumbling along in the background).