This page has a list of development kits and test cartridges for various 8-bit era consoles and handhelds, including Gameboy and Gameboy Color. The top picture is the so called WideBoy which is a device that you plug on top of a FamiCom. It duplicates the image to the TV attached to the FamiCom to relieve the eyes of the poor developers. The bottom picture is a funny looking cartridge which is in fact just a simple EPROM cartridge.

Thanks nordloef for the tip.