Full Screen Mario
Fully playable in-browser Super Mario Bros game written in HTML5 which allows you to design and save your own custom levels (as well as generate random ones as well):
Full Screen Mario is a fully 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.
Try it out for yourself here (Chrome only)