Full Screen Mario is Josh Goldberg's complete remake of the classic Super Mario Brothers in HTML5. You can play re-creations of the original levels, make your own in an HTML5-based level editor, or play any of an infinite number of randomly generated levels. The code is on GitHub for your happy hacking, too. It's a pretty impressive example of what HTML5, in-browser functionality can do.
Full Screen Mario is a purely HTML5 remake of the original Super Mario Brothers. You can play the original levels, play through some of literally millions of possible random maps, or create your own using the level editor. This whole project is open source and free – if you would like to use the code, check out the Github or email me.