The Atari 2600 has seen ports of Donkey Kong, Donkey Kong Jr., and the original arcade Mario Bros., but that wasn't enough for Sprybug over on the Atari Age forums.  He's busy putting together a version of Super Mario Bros. for the 2600, complete with background music and boss battles!  Sprybug has released a demo of a limited version of world 1-1, and he's working on boss battles against the nefarious Bowser Jr..  Take a look at the demo footage and see what's possible on the 2600.

Demo Footage: (Youtube)

Source: AtariAge Forums