Commodore 64 4 Player Joystick adapter
Simple plug this into the user port of your Commodore 64 and run one of the supported games. More games are being updated to support 4 players.

Some of the games supported are

Alone in the Green
Amazing Maze (3Players)
Bombmania
Garrison (3Players)
Hockey Mania (Demo)
IK+ Gold (3Players)
Marble Madness
M.U.L.E.
Phong!
Quadris
Quadtron
Rampage Gold (3Players)
Snacks 4 Snakes
Space Lords
Square Attack
Ivan 'Ironman' Stewart's Super Off Road (3Players)
Tanks 3000 (Preview)
Team Patrol (PD Demo)
Tour de France 2005