Tactician (set 1)

At its core, Tactician might seem like another Space Invaders / Galaxian clone - you control a small ship at the bottom of the screen, which can actually move up or down a bit, instead of just left and right - but it has a pretty cool gimmick going for it.

At the beginning of each stage, you can "draw" a barrier by placing little dots around the playing field. After several seconds, enemies are released from their bases and begin to swarm the screen. They'll usually make quick work of the barrier, but if you shoot the red ball it'll explode and destroy anything near it. Quite a cool concept for something from 1981 - it's a shame that Tactician is so unknown, and that the idea was never expanded on in any other games. Some versions list the game as copyrighted by Sega, but they apparently only distributed the game in certain territories.