Game Boy Camera and PrinterThe Game Boy Camera was a cartridge which includes the first smallest portable digital camera:
Here is a special golden Zelda version
Game Link CablesThe Game Link Cable was used to have serial communication between the two game boys for things like multiplayer:
Cheat and Save-Backup CartridgesThere were a few different cheat cartridges which would let you plug your own cart into and then by some mechaism affect parts of the game.
Here's the Game Genie:
Other popular cheat carts are the game shark (which would hook into the vblank interrupt handler to patch RAM at every frame) and the action replay:
There were also Xploder and MonsterBrain
- http://gallowsmart.wordpress.com/tag/mega-memory-card/ and color cart II