A SNES cartridge released in 1994 by Nintendo.
This cartridge simply allowed Game Boy games to be played on SNES. The main advantage, of course, was that the screen size was much bigger and the picture quality was much better (remember, this was way before Game Boy Color came out).
Also, the screen colors could be chosen, so the whole game could be pseudo-colored. Also, AFAIK some games included the game coloring information - the game worked in color on SGB and grayscale on normal GB.
As far as I know, this wasn't that much of a success, but many games did support SGB (it is not emulated in modern systems, though). After this thing, Nintendo did it right with Gameboy Color, a GB-compatible system that also had more power.