The most popular IC ever made. Switch between astable (continuous oscillator) and monostable (one-shot pulse) modes. Frequency, period, duty cycle, and timing — all live.
The 555 timer was designed by Hans Camenzind in 1971 and is still produced by the billions every year. It can run as an astable multivibrator (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 is 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.