At first glance you'd think the site is a bit fishy with its very basic appearance but Nintendo8 is a great place to relive your classic gaming experiences. It includes tons of 8 bit NES games that you can play absolutely for free online through your computer with keys on your keyboard functionining as the controller buttons.
Some games available include Contra, Legend of Zelda, Super Mario Bros 1-3, Bomberman, Dr. Mario, and more.
The site has even expanded to include newer games for various other systems. Commodore 64 games available include Ghostbusters, Wonderboy and Boulder Dash. They've introduced Gameboy games like Pokemon Yellow or Silver and games like Wario Land 2 and Killer Instinct. Sega Master System games like Sonic the Hedgehog, Phantasy Star, Mortal Kombat and Golden Axe are also there as well as a few other popular retro titles from the 1980s and 90s.
Sadly since they're just flash games you can't save your progress so once you leave the site your game is lost and you'd have to restart from the beginning to play again.