There's a huge online community dedicated solely to creating games for outdated consoles. One adventurous user on the AtariAge forum has come up with a version of Super Mario Bros. made for the Atari 2600.
Joystiq brought this to our attention earlier today, and we're pretty impressed. It's the handiwork of one "Sprybug", who explains in the forum thread at AtariAge that he's hoping to keep adding to his port. Watch it in action below, or download it here.