Multiple Cartridge Selector for the Atari 2600 Game Console - Dismantled for reference and curiosity
Released in 1981 the ROM Scanner produced by Marjac was an add-on that sat above your Atari VCS / 2600 and let the user select from ten inserted games at the press of a button without having to power down the console.
Sounds a bit clunky and basic however at the time it was pretty neat. It was powered by the 2600 and let you swap between games in a second. The system automatically reset to load the new game when a button was pressed.
Using the ROM Scanner was convenient and had the added bonus of greatly reducing the wear and tear on your 2600's cartridge port. It also removed most of the pain of plugging and unplugging the CompuMate Keyboard made by Spectravideo (see other page on this site for details and example image below).
The only real downsides are that a few of the more odd shaped cartridges didn't fit into the slots, like the XONOX Double-Enders and while it did work with the 2600 Jr. it didn't sit nicely on top as it did with all 2600 classic models.