Atari was a real trailblazer back in the '70s and '80s arcade landscape, and was responsible for creating some of the most famous video games of the period – and it almost published a game which would have pre-dated the likes of Beatmania and Dance Dance Revolution by over a decade.
Planned for launch in 1985, Jammin' was never released and was assumed lost, but a prototype was recently recovered from old VAX backup tapes by Dutchman2000 and SynaMax – and is now in a position where it can be played in MAME.
Interestingly, the original prototype of Jammin' was placed inside a Donkey Kong cabinet and was designed to use the Zilog Z80 microprocessor, which suggests Atari intended it to be a conversion kit that would have allowed arcade owners to install it in existing DK cabs and therefore gain a new title cheaply.
You can read more about the game's discovery in this thread.