Galaxian
The Game: In one of the most seminal variations on the Space Invaders format, Galaxian was among the first clones to introduce attacking formations that would break off from the usual rows and columns of 
invaders. Though Galaxian‘s use of this innovation was minimal, it was a drastic change from the usual slowly-advancing target gallery. (Bally/Midway [under license from Namco], 1979)
Memories: Galaxian may not be as well remembered as the much more strategically challenging Galaga, but it ultimately added a vital new twist to the Space Invaders-inspired genre, a format which was badly in danger of becoming stale. Galaxian was also the first arcade video game to use a color display instead of a monochrome monitor with translucent colored overlays. [read more]
The Game: Those pesky invaders from space are back, and this time they’ve devised a handy delivery system that drops entire columns of kamikaze invaders and motherships through a series of airborne chutes from an orbiting Stern command ship. Players can try to intercept invaders as they plummet toward Earth, but as their impact sends a cloud of debris spreading outward which can also destroy the player’s cannon, avoidance is a perfectly acceptable (if not exactly high-scoring) survival strategy. (Stern [under license from Union Denshi Kogyo Company], 1980)
The Game: It’s quite simple, really. You’re the pilot of a ground-based mobile weapons platform, and there are buttloads of alien meanies headed right for you. Your only defense is a trio of shields and a quick trigger finger. If your cannon is destroyed, its pilot must run for cover; each of the three shields contains an extra cannon. When all three shields are gone, the alien commander – a kind of spaceborne crab – will descend to nearly ground level and hunt the helpless pilot down. Ten rounds of this decide the outcome of the war. (Magnavox, 1980)
The Game: It’s quite simple, really. You’re the pilot of a ground-based mobile weapons platform, and there are buttloads of alien meanies headed right for you. Your only defense is a quartet of shields which are degraded by any weapons fire – yours or theirs – and a quick trigger finger. Occasionally a mothership zips across the top of the screen. When the screen is cleared of invaders, another wave – faster and more aggressive – appears. When you’re out of “lives,” or when the aliens manage to land on Earth…it’s all over. (Texas Instruments, 1981)