A Brazilian romhacker by the name of FiguiDOS has build a recreation of Sonic CD on the codebase of Super Mario World, utilizing both the SA-1 and MSU-1 enhancement chips. It's definitely not a faithful recreation, but still an impressive feat regardless.
A 16:9 video of a 4:3 game. Sacrilege. As for the game itself, it looks more like a fan game based on the Master System version of Sonic the Hedgehog.