Commodore 64 joystick adapters

Commodore 64 joystick adapters are hardware peripherals that extend the number of joystick ports on the Commodore 64 computer. The additional joysticks can be used on games with dedicated support for the specific adapter.
A number of different joystick adapters have been constructed for use with the C64. The "Classical Games" / "Protovision" adapter is by far supported by the largest number of games. While building instructions are available for most of the adapters, a few adapters are also available commercially. The adapters are also emulated in some of the C64 emulators.
Original Adapters.
Starbyte adapter (1990).
Supports 2 additional joysticks at the userport. Released by "Starbyte Software" in 1990 for use with their game "Adidas Championship Tie Break". At the time sold separately under the name "Tie Break Adaptor".
Building instructions (plugs and wires only) have been reverse engineered and made available by Groepaz/Hitmen.
Kingsoft adapter (1992).
Supports 2 additional joysticks at the userport. Introduced with the game "Bug Bomber" by "Kingsoft" in 1992. The game came with instructions to build your own 4-player adapter.
Building instructions (plugs and wires only) have been reverse engineered and made available by Groepaz/Hitmen.
Protovision 4 player interface / Classical Games adapter (1997).
Supports 2 additional joysticks at the userport. Designed by "Chester Kollschen" ("CKX") of "Classical Games" and released in 1997 with the game "Bomb Mania". Kollschen later joined "Protovision" and the adapter is now available as the "Protovision 4 player interface".
Building instructions (plugs, wires and an IC), sample code, shop and list of supported games are available at the Protovision website.
Space Balls adapter (1998).
Supports 8 joysticks, all connected to the user port and one joyport. Designed by "Luigi Pantarotto" and released with the game "Space Balls" (1998) for 2-8 players.
Building instructions (plugs and wires only) is provided with the game "Space Balls" (called "Star Balls" in its schematics page).
Ninjas SNES pad adapter (1998).
Supports 8 SNES pads at the joyports. Designed by "Ninja" of "The Dreams".
Building instructions (plugs and wires only) are available at the "Hitmen 4 player adapter" website. Code interface is demonstrated in the "SNES-Pad-Tooldisk #1".
Digital Excess & Hitmen 4 player joystick adapter (1999).
Supports 2 additional joysticks at the userport. Designed by "Groepaz" of "Hitmen" and "Thomas Koncina" and "Bjoern Odendahl" of "Digital Excess" in 1999 as an easier to build alternative to the "Classical Games" adapter. 
Building instructions (plugs and wires only) and sample code is available at the "Hitmen 4 player adapter" website.
Inception - 8 joysticks adapter (2013).
Supports 8 joysticks at one joystick port. Designed by Ray and PCH (Petr Chlud). 
Available commercially at the c64.cz website.
Compatible Adapters.
Singular Crew 4Player Adapter (2008).
Supports 2 additional joysticks at the userport. Compatible with the Protovision 4 player interface / the Classical Games Adapter, Digital Excess & Hitmen 4 player joystick adapter and the Kingsoft Adapter. 
Designed by Singular Crew. 
Available commercially at www.zsibvasar.hu website.
Related Adapters.
Adapters for a few related computers can also be used with the C64. Those adapters include "PET" (normally only used on the PET/CBM2), "Hummer" (normally only used on the C64DTV) and "OEM" (normally only used on the VIC20). There are probably no game supporting those adapters on the C64.
Emulation.
The C64 emulator VICE supports the "Classical Games / Protovision" and the "Digital Excess & Hitmen" adapters from version 2.3., and the "Kingsoft" and "Starbyte" adapters from version 2.4.
Supported games.
A (near) complete list of games with original or added support for the various adapters.
Adapter:
"S = Starbyte, C = Protovision / Classical Games, D = Digital Excess & Hitmen, N = Ninjas SNES pad, K = Kingsoft, B = Space Balls", I = Inception
Description:
"original/clone = new implementation, patch = patch for existing game, hack = unauthorized release based on existing game."
Interface origin:
"Publisher or official source of joystick adapter support. In order to use hacked versions legally, you should own the original game."