The Spectravideo Compumate was the only keyboard add on ever released for the Atari 2600. It featured BASIC in ROM and had the ability to load and save to audio cassette tapes, though you needed a separate tape recorder. In addition to BASIC programming, the Compumate also had some nice musical ability, doubling as a musical keyboard basically.
It featured a 42 key qwerty keyboard, but the keys were the flat plastic type, not clicky keys, which limited typing speed severely. The keyboard attached to the Atari 2600 via a cartridge. In and out was limited to the microphone and headphone jacks located on the keyboard: it could pipe both data and sound out of either one of those. There was no way to add a printer or any external drives, except the cassette tape player.
You can view a good picture of this system at
This item is worth about $75 USD without the box or instructions, or about $300 for a complete unit.