Super Mario fully playable online using HTML5
Full Screen Mario gives you the game, but also lets you create your own levels online.
You could plug in your old Nintendo Entertainment System, load up a vintage Super Mario Brothers cartridge, and play one of the greatest games of all time. Or you could just pop over to Full Screen Mario and get your fix online.
Josh Goldberg is behind this perfectly reproduced Super Mario remake done entirely with HTML5. If you're looking to switch things up and you like surprises, then you can play through randomly generated levels. Purists can stick with recreations of the levels from the original game.
Some of the random levels are pretty challenging. One opened up with a barrage of cannon fire that took me out before I got halfway across the screen.
The open-source endeavor also lets people build their own levels with an online level editor. You can squeeze as many question-mark boxes in there as you wish.
You can kiss your productivity goodbye the moment you click over to Full Screen Mario. Not only does it give you the opportunity to revel in retro gaming, but you will also entertain yourself with endlessly building levels full of nothing but springboards, just like you dreamed about when you played Mario as a kid.