The 555 timer IC can be used with a few simple components to build a bistable circuit, also known as a 'flip-flop'. It is a very simple memory circuit.
The circuit is called a bistable because it is stable in two states: output high and output low.
The bistable circuit has two inputs:
Trigger (pin 2) makes the output high. Trigger is 'active low', it functions when < 1/3 Vs.
Reset (pin 4) makes the output low. Reset is 'active low', it resets when < 0.7V.
The power-on reset, power-on trigger and edge-triggering circuits can all be used as described for the 555 monostable circuit.
Recommended book: IC 555 Projects
Rapid Electronics: NE555 timer
Next page: 555 Buffer