I do believe we are all comfortable in the presence of Interface 2, aren't we? Oh, I see. Well, it was a smart add-on Sinclair released for the ZX Spectrum in 1983 and offered two joystick ports and a cartridge slot, effectively turning the Speccy into a proper console. Or, at least, that was the plan, but the joystick ports weren't Kempston compatible and the cartridges were both very expensive and able to only hold up to 16k worth of ROM, meaning that this upgrade didn't really make a lot of sense. Still, the Interface 2 remains a lovely and absolutely beautiful collectible that did have a few excellent games released for it.
You can grab a complete, boxed one with manual via this Sinclair ZX Interface 2 eBay auction. The thing seems to be in great condition and the seller ships to the United States, the European Union, Australia and Canada.