SEGA does what Nintendoes.

Someone, somewhere has ported Super Mario Bros. to the Mega Drive! Now, this one is much more capable than your average pirate knockoff.

It looks and almost sounds exactly like the NES original Super Mario Bros., a game that has been ported to many platforms. This port, created by Argentinian programmer "Mairtrus," is probably faithful for a very good reason. According to the Mairtrus, an automatic converter did much of the work, swapping one processor assembly language for another, leaving him with the chore of optimizing the game down to 64KB.

"At least the 75% of the code is still untouched after the translation," Mairtrus writes.

SOURCE (there's some other videos on that link that showcase comparatively-bad pirate games).

Personally, I thought it looked spot-on, and you could only tell it wasn't on NES because some of the music didn't sound quite right. I thought the underground theme kicks ass on the Mega Drive, though! Guy says he's done/doing a new version which will run on actual MD hardware. Hmmmm, pointless-but-cool!