Star Castle Retro Flash Game
This is an old, but classic 80s game which used vector graphics. Although not as well-known as the other games on this site, it is both simple to play and very addictive.
Originally created in 1980 by Cinematronics, the object of the game is to obliterate a series of defenses circumferencing a stationary turret in the center of the screen. Just use the arrow keys to move your ship and the spacebar to fire.