The second-ever videogame. From the YouTube page:
Way back in 1958, William Higinbotham invented Tennis For Two to liven up visitor day at Brookhaven National Laboratory, his workplace. The game uses an oscilloscope with two control pads. It remained largely unknown until 1981 when a lawyer trying to break Magnavox's patent for video games came across writings talking about the game.(via b3ta)
Blueprints of it were found to predate Magnavox's game, the case was settled out of court, and the game found fame as the second ever invented, since it was later predated by A.S. Douglas' 'OXO' game from 1952.