The GAU-8, properly known as AN/GAU-8 Avenger, is a 30mm fully automatic 7-barreled heavy autocannon resembling a scaled-up version of the M61-A1 Vulcan. The entire weapon, outside its housing, is the size of an SUV. It was designed by General Electric and Philco Ford for the United States Air Force in 1971 as the primary weapon of their new combat air support plane, the A-10.
It fires 3900 rounds per minute at full cyclic rate, which equates to 65 rounds per second. Considering that each bullet is roughly the size of a large pill bottle and weighs nearly a kilogram, this cyclic rate is fairly impressive. (Generally, the cyclic rate of a weapon varies inversely as its caliber). While the gun was initially designed for use on the A-10 attack plane for destroying tanks, it is also very effective at killing helicopters or fighters that get too close.
The Dutch also found an interesting use for the GAU-8: They mounted it on a motorized turret equipped with radar, and used it aboard ships as a defensive weapon. This configuration, known as Goalkeeper is considered by many to be more effective than the US-built Phalanx system, a similar defensive weapon based on the M61.
Thus far, no aircraft other than the A-10 has ever carried this weapon.
Thanks to toalight for clearing me up on exactly when the GAU-8 was initially built, and by whom. (I had initially said 'Late 1970s' here - clearly off by a few.)