The Pixelation forum ran a contest for users to create screenshots of what modern games would look like on the Gameboy. As you can see, that's BioShock, and there's a pretty good Twilight Princess and Guitar Hero after the jump. The board's current contest is to make your own Mega Man level, within the color and graphic limitations of the original NES games. Pretty neat stuff, and there's definitely a good amount of talent out there. Check it out and give it a go if it's your cup of tea. If it's my cup of tea, slip in a roofie when I go to the bathroom. I'm trying to get crunked.
Hit it for the other two mock-ups.