Were you planning on doing any work the rest of today? Too bad. All of Super Mario Bros. is on the internet for you to play.
Full Screen Mario is an HTML5 remake of the original game. You can play all the way through, or jump to any level you'd like. And if you are feeling REALLY bored, you can even build your own levels. Have fun doing this and nothing else the rest of the day.
[H/T Laughing Squid]