555 Timer 

The 555 is a timer chip with many applications.
It is a very rugged and robust chip with 300mA drive from output pin 3.
It is not suitable for use in circuits that use gating chips as it produces a lot of interference on the supply rail.

For a complete discussion on this chip, see the subscription section of: 
Talking Electronics Interactive.

Circuits using a 555:

Producing a negative supply