Spacewar!

Spacewar! is one of the earliest known digital computer games.

Steve "Slug" Russell, Martin "Shag" Graetz and Wayne Witaenem of the fictitious "Hingham Institute" conceived of the game in 1961, with the intent of implementing it on a DEC PDP-1 at the Massachusetts Institute of Technology. After Alan Kotok obtained some sine and cosine routines from DEC, Russell began coding, and by February 1962 had produced his first version. It took approximately 200 hours of work to create the initial version. Additional features were developed by Dan Edwards, Peter Samson and Graetz.

Source: Wikipedia, "Spacewar!," available under the CC-BY-SA License.

"The Origin of Spacewar!" by J. M. Graetz, published in Creative Computing Video & Arcade Games Vol. 1, No. 1, Spring 1983, provides an in-depth look at the development history of the game.

Spacewar! may be played via browser here as of October 2011:
http://spacewar.oversigma.com/html5/