Commodore 1351 Mouse
The 1351 was a mouse that looked identical to the mouse for the Commodore Amiga, however it was not compatible with the Amiga. It could be put into a joystick emulation mode by holding down the left mouse button as you plugged it in.
This mouse made using Geos feel like using a real computer (like an Amiga, or a Macintosh). It was a must have device for those of us living in denial that our Commodore 64s were at the end of their useful lives.