Galaga

Galaga was released as a sequel to Galaxian in 1980 by the Bally/Midway corporation. This game featured many things that Galaxian did not- the enemies flew in from out of the screen, and the most memorable thing of all was the ship that came down and could capture your ship with it's tractor beam. The dual ships that ensued after you reclaimed your captured ship was a never before seen feat. Seems like nothing these days, but back in the 80s Galaga was a huge hit, and even gets recognition today. It was a huge breakthrough and allowed tons of base for shareware games of the 1990s. Look for it in arcades, on Midway Classic Series of Games, or on the re-releases of Atari Classics.