The game:
Japanese company Taito had the inspired idea around 1978 that killing aliens was extremely good fun. So they invented a game called Space Invaders, lots of history then followed, some people got rich and so on...
It's a simple game with simple graphics, but it has one of the most vital assets for any computer game — gameplay.  Modern games seem to forget this with their fancy "3D graphics" and "millions of dollars".  All you need is a pointy green thing which shoots white stuff at a rampaging invasion of wobbling blobs.  They don't make 'em like they used to.

How to play:
Left arrow - Move left
Right arrow - Move right
Space - Fire laser
P  - Pause/unpause game
Q  - Quit game
S  - Small scale version