From MAME Team:
MAME, short for Multi Arcade Machine Emulator, lets you relive the golden age of the video arcade right on your PC. Instead of playing copies or clones of arcade games, MAME lets you play the actual arcade game. A variety of games is available for demo evaluation downloads, including Crystal Castles, Burger Time, Joust, Xevious, Donkey Kong, Tempest, and of course, Space Invaders.
Note: In order to use this emulator, you've got to find the original games (also known as arcade ROMs) that run with this arcade emulator by searching on the Web. They cannot be included with this download for legal reasons.