555 Timer IC

Is an integrated circuit used in a variety of circuits, pulse generation, and oscillator applications. They can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package.

Introduced in 1972 by Signetics, the 555 is still in big use, because it´s easy to use, low price, and good stability. It is now made by many companies in the original bipolar and also in low-power CMOS types. 

As of 2003, it was estimated that 1 billion units are manufactured every year.