[Damian Peckett] has implemented an Apple ][, its 6502 processor, and a cassette port, all on an Arduino ... The 16u2 is normally used as the Arduino Uno’s USB interface. The only external ...

This feature isn’t exclusive to the Leonoardo, as [Michael] explains in a build he sent in – the lowly Arduino Uno can also serve as a USB HID keyboard with just a firmware update.