"The first CRT display was a converted oscilloscope used to play SpaceWar.The first trackball (and thus, the first mouse) was a SpaceWar control at MIT. It is said that Ken Thompson salvaged a PDP-1 and created a new operating system, now called UNIX, so that he could play SpaceWar.
The gameplay was inspired by E. E. "Doc" Smith's Lensman novels. Two players go head-to-head, each controlling a ship in interstellar combat, trying to blow the bejeezus out of each other. There is a sun in the center of the playing field that exerts an inverse-r-squared force on all objects on the screen. A talented player can aim torpedos such that their trajectory is deflected by the sun's gravitational force until it intersects with the other player's ship."
This PC version by B. Seiler is very faithful to the original, down to the black-and-white vector graphics. You can toggle gravity pull of planet on or off, and play against another human player or a computer player. All this on 9KB no less. Whether or not you are interested in the history of computer games, SpaceWar is one classic that you wouldn't want to miss.
Note: As a bonus, you can download the 1998 Windows remake of the game in the "Extras" area below. Created as a remake of the 1979 Cinematronics version, only two player mode is supported.