The most popular IC ever made. Switch between astable and monostable modes. All values live.
The 555 timer was designed by Hans Camenzind in 1971 and is still produced by the billions every year. Runs as astable (continuous square wave), monostable (single pulse on trigger), or bistable (flip-flop).
Astable: tH = 0.693(R1+R2)C, tL = 0.693·R2·C. Duty cycle always > 50% in the classic topology — for 50% duty you need a diode trick.
Monostable: pulse width = 1.1 · R · C. Trigger the input, get exactly one pulse out.