Space Cavern is another game that is similar to Space Invaders.  You are the commander of an intergalactic starcruiser in an uncharted quadrant of outer space...but to me it looks a lot more like you are Luke Skywalker in the outfit he wore in the first Star Wars movie.  You run left and right at the bottom of the screen as aliens (called Electrosaurus) shoot at you from above.  What makes this game different from many of the other Space Invaders type games is you are also attacked from the sides (by Shaggy Marsupods).  You press the red button to shoot up, move the joystick left or right to move left or right, move the joystick up to shoot left and move the joystick down to shoot right.  When you are hit, "your skeleton will glow inside your body as the bio-molecular components of your body disintegrate" (according to the instruction manual).  Although this sounds a lot cooler than it is, it was pretty advanced for a video game to even try to provide this kind of image for a character death.  You start out with four men.  You get an extra man every  20,000 points but you can never have more than four men at one time.  Although this game is a bit more difficult to find than Space Invaders, it is still relatively common and can be found for pretty cheap...probably because it looks like it would be cool but isn't really very fun to play.

Screen Shot from Instruction Manual