Super SADX
From Sonic Retro
Super SADX is a technical hack, created by Endri of the PC version of Sonic Adventure DX which allows Sonic to become Super Sonic like the classic sidescrollers on the Genesis.
Just like the classics, Sonic can transform into Super Sonic by collecting 50 Rings. Unlike the games, he has to simply beat Super Sonic's story to become super, versus collecting the Chaos Emeralds via Special Stages. (Though the Emeralds being collected in the story probably counts as the same thing) There are restrictions, he only works in Action Stages, meaning you can't traverse the hubs as Super Sonic.