Wikipedia
GameBooster is a non-licensed cartridge for the Nintendo 64 made by Datel Design & Development Ltd. It was made to be similar to the Super Game Boy for the Super NES for playing Game Boy games on the Nintendo 64. It was released halfway through the console's time in the Gaming Market.
It plays most Game Boy games as well as working with Super Game Boy games with borders. By pressing both the L and R button will turn on the menu page to increase the screen to full as well as changing colour to the screen.
Because it's not licensed by Nintendo, a slot was added at the back whereby putting a Nintendo 64 game into the slot would override the lockout chip.
One problem with the Gamebooster is that it doesn't play any sounds from the Game Boy game in play, instead plays a tune over and over, being one of the main problem with the game cartridge.
Even with no Game Boy game inserted, the cartridge has a built in game called Rebound Mission.