SpaceWar may be the most important computer game ever. The first version was developed for the PDP-1 at MIT in 1960. The game has been under essentially constant development since.
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.
Modern versions of SpaceWar include several galaxy-wide, unlimited player games over the internet.
In 1977, Cinematronics created a stand-alone version of SpaceWar. I have one. It's the size of a large refrigerator and it occupies perhaps 20% of the space in my apartment. When a ship explodes, the window rattle.
I've been programming SpaceWar for the Macintosh. You can play against the computer or against another human being. Eventually you will be able to play over the net. If you don't have a Macintosh, I can't help you.
Development has been put on hold but you can play with the final beta if you like. This requires Apple's NetSprockets or else it won't run. You can download NetSprockets and the other GameSprockets for free from Apple.