Not much to add really. Midway has gone all socialist and is providing the masses of the Internet with freeware versions of Defender, Robotron 2084, Tapper, Spyhunter and Joust (to name a few). All you have to do, is make sure you've got Shockwave installed (get Shockwave) and visit their great (virtual) arcade.
Relates @ Gnome's Lair: Online MSX emulation, Head over Heels, free C64 games
Related Tags: Retro, Game, Games, Free, Freeware, Midway, Vintage, Retro gaming, Video games, Robotron, Defender