TheSmartAss.info's suite of Java emulators allows smooth, in-browser playback of literally thousands of old-school video games: 517 Atari titles, 148 for DOS, 636 Game Boy games (and 410 for Game Boy Color), 2,019 (!) NES titles, 238 GameGear games, 802 Sega Genesis titles, and 284 for the Sega Master System. Highlights include Space Invaders, Frogger, Galaga, Pitfall!, Super Mario Bros., The Legend of Zelda, Metroid, SimCity, Zero Wing, Duke Nukem, Sonic the Hedgehog, Aladdin, Earthworm Jim, Pokemon, and Metal Gear Solid. Use the search function to find your favorites! You can also register an account to save games on emulators that support it. Make sure to check the purple bar below each game for control info and links to alternate emulators in case the default one is buggy or slow.
Interested in video game emulators? Yearning for the games of yesteryear? There is probably an emulator for any 'classic' game that you could wish to play. General arcade emulators, SNES, NES, Genesis, Atari 2600, et al. For the truly curious, listen to the story of the emulator that almost was, Silhouette, the emulator Nintendo almost released.