Sonic Retro is home to a great many Sonic 1 hacks, many of which don't really change much. However, you probably all know of a few that do perform substantial changes, like Sonic Megamix and Sonic 1 With Fries. Sadly, one of the most ambitious Sonic 1 Hacks was never released.
This was to be a game called "Dustin Wyatt's Epic Adventure", a Sonic 1 ROM Hack starring Dustin "Tweaker" Wyatt, well-known Sonic reverse-engineer, as he battles other reverse-engineers throughout varied stages in Mega Man style. Though the game was heavily hyped up, it was ultimately never made available to the public, and the only remaining info on the game is in the form of screenshots, a few video clips, and one piece of concept art.
Even though this game was never released, it still stands as a testament to what Sonic hacking can do. I personally want to play this game myself.