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.

Mine is a CGB-EMULATOR (Color Game Boy), but the sticker on the bottom also indicates that it has Game Boy Advance options. I'm not sure if it can do both, or if this is a GBC Emulator modified to emulate the GBA instead.

Here's a close-up of the cartridge interface. I believe you can use a normal Game Boy Color with this, but I'm not sure. (The white plug I think just provides power to the special Game Boy that came with the system.)

Here's the main circuit board of the unit itself. (CGB EMU CARTRIDGE ADAPTER2(B) is printed on the little daughter board.)

Also note that there's 32 Meg of memory via normal computer SIMM module: