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.
The Retroist is a veritable treasure trove of 80's (and 70's) goodness. TV commercials, catalogs, and of course the poetry of Mr. Leonard Nimoy. The Youtube channel alone is worth the price of admission-- Tobor! Diet Rite! Candyland!
It's the 1981 Atari product catalog!
Steem is an Atari ST emulator for Windows and Linux that is very simple and user-friendly. More details on installing are in a helpful beginner's guide, but you're probably most interested in the games, of which there are lots [more inside].