Information: Taken form the Harmony manual.

The Harmony cartridge is a programmable add-on for the Atari 2600 console that allows you
to load an entire library of games into a single cartridge and then select which title you want to
play from a friendly, on-screen menu interface. It features an SD card interface, making it simple
to access the large library of Atari 2600 software. The Harmony cartridge supports almost all of
the titles that have been produced for the Atari 2600. It can also be used to run your own Atari
2600 game creations on a real console. The Harmony cartridge is flash-upgradeable, and will be
updated to support future Atari 2600 developments.

I have not had time to play around with it yet but soon enough I will be playing some combat on a 13″ tube TV and loving every minute of it.