The CompuMate by Spectravideo was a rather unique add-on for the Atari 2600 console, that almost turned Atari's all conquering games machine into a home micro. It added a keyboard that attached itself to the 2600, some extra RAM, a version of BASIC, a simple music synthesizer program, a rudimentary drawing utility and even provided with the option to connect to a tape recorder.
As the accessory didn't fare all that well it is quite rare, but you can easily grab one for a reasonable price. This boxed CompuMate Spectravision on eBay seems like a pretty decent offer. It does after all come in its shiny original box. Seller ships only within the US.