If you somehow have a working Nintendo NES clone—or an actual NES you don't want to turn into an HTPC, "bunnyboy" of the Nintendo Age forums has a proposition for you. Pay him $130 (inclusive of shipping), and he'll send you a custom-made NES cartridge that runs GameBoy cartridges.
Why you'd want to pay a significant sum despite (or perhaps because of) the retro-friendly tech is a mystery, but bunnyboy pulled out all the stops with the NES RetroVision. His "incredibly amazing custom board" reads the GB cartridge, converts all signals into NES format. To properly read information from the 1P NES controller, the process is reversed. The 2P controller becomes a configuration tool of sorts, allowing users to toggle through available colors for the screen border and the actual monochrome GameBoy graphics.
bunnyboy's creation even includes a link port, allowing multiplayer with another GameBoy—or even another RetroVision unit. For audio the options to plug in a set of earphones into RetroVision's headphone jack, or allowing the NES itself to handle the audio through some sort of unspecified modification on the gaming console.
Last but not least, the NES RetroVision has transparent casing. No doubt to help bunnyboy showcase his expert fabrication skills. While he's aware that his creation's use of an obsolete platform will limit sales, he's confident that developments that made RetroVision possible will be useful for future "secret" projects.
FS: NES RetroVision - $130
See Also: Back to the Future - 15 Retro Gadgets