Autoloot was introduced in Patch 1.9. If you <Shift>+click on a mob, chest, or other container that has items, you will automatically loot all non-bind-on-acquire items. You can also use the same method to automatically skin an animal, mine a vein, and harvest some herbs. This is now settable as an interface option as of Patch 2.0.1, so that you might default to autoloot on a right click, without having to shift-right click to loot manually.
AutoLoot used to be an addon for Cosmos. It was removed once Blizzard introduced the <Shift>+click method of autolooting.