This is the Color Game Boy Emulator from Intelligent Systems (the company that makes most of Nintendo’s official devlopment hardware). If you go to their website (it’s in Japanese) you can see the current versions of this type of hardware for the Nintendo DS, Gamecube, GBA and this GBC unit.
It connects to a computer via a SCSI interface and comes with software to code games and then transfer the game image to a Game Boy connected to the Emulator.
The sticker on the bottom indicates that it has support for 3 options:
CGB-DEBUGGER : This tool can be used to transfer, execute, check and debug user programs on Game Boy Color.
AGB-EMULATOR : This tool can be used to transfer, execute and check user programs on Game Boy Advance. (It can also be used for the development of Game Boy and Game Boy Color software.)
AGB-DEBUGGER : This tool can be used to transfer, execute, check and debug user programs on Game Boy Advance.
Same seller also has a Wide-Boy 64. These were used by licensees to test their GB, GBC, and GBA games on a TV screen or monitor, rather than the screen of said portable systems. They also have a Nintendo GameCube NR Reader in amazing shape. This onsole used to develop and debug games for the Nintendo Gamecube.