My old 8051-based keyboard wedge.
And apparently the one I prototyped on my C64 as well!
It sat on the PS/2 bus between the keyboard and PC, and connected to old microswitch joysticks. Joystick actions were translated to keystrokes so old DOS games could be played retro style (yes, there was a retro even back when DOS was a thing). Though I doubt I was using DOS at all by the time I was done tinkering with it.
The C64 version plugged into the user port and was just an edge connector with a chip and some wires soldered to it... and a piece of string so I could pull it out without bending anything.