I got this Sonic 1 hack the other day, called Sonic 1 Megamix. You get new controls (spin dash, homing attack, double jump), and all of the levels have been completely redesigned. Best of all, you can play as not only Sonic, but as Mighty, Shadow, Tails, and Knuckles.

I'd provide a link and tell you to play this masterpiece right now, but I'm pretty sure it's illegal.