This cartridge looks like an EPROM cart, but I believe it's designed more for testing
various parts of a Game Boy. It's called a Multi Checker (Part# DMG-MBC1).

This is a simple EPROM cartridge. You just plug in your EPROM and go! Game
companies would send these out as review copies, so they would sometimes attempt
to pretty them up a little with labels and such.
Part# DMGC-MBC1-2M-EPROM1-01

Getting more advanced here, this is a 32M Flash Cartridge. You would plug the cart into a special cartridge programmer, and it would just upload the game to the flash chip. These also have a little on-board RAM so you could test game saves and such.
Part# DMG-MBC5-32M-FLASH (DMG-B04-02 on circuit board)

Pretty much the same cartridge as above, but this one has the 'Rumble' feature installed so you can test your games that vibrate. A AAA sized battery goes under the black cover.
Part# DMG-MBC5-32M-R-FLASH (DMG-B03-11 on circuit board)

Here's the first generation of the Game Boy Advance Flash cartridge. 64Mbit storage, and it would appear that this could be increased by adding more RAM chips (although I've never seen one of these with more than 2 chips).
Part# (is in Japanese) (AGB-F01-01 on circuit board)