The Mega Switcher for the Commodore 64 plugs into both joystick ports on the C64.
This allows you to use the Sega MegaDrive controller on the C64 (buttons B and C as primary fire and secondary fire buttons or vice versa) and allows changing which Joystick Port is being used without unplugging the joystick.

Also works with 8bitdo-dot-com's wireless Genesis/Megadrive controllers.

For more information link to open source project (also contains instructions about how to use the configuration switches):

https://github.com/tebl/C64-Mega-Switcher