The Sonic fanbase is known for being one of the largest (and most divisive) on the internet, to the point where several games have been ported to PC or the Nintendo 3DS; the following is a list of random fan works (excluding Sonichu to avoid vandalism).
Sonic.exe and Sally.exe - A pair of fan games that remake both creppypastas.
Sonic.exe: Nightmare Beginning and Sally.exe: Continued Nightmare - Two fan games by JaizKoys that adds more story and gameplay than just running to the right, including the ability to save the characters.
Sonic Green Snake - A rather infamous hack of the original Sonic the Hedgehog, notable for being released in an extremely incomplete state and getting the creator banned off the Sonic Stuff Research Group for several years.
Sonic 2 Delta - A series of ROM hacks by Portuguese hacker Esrael, the latest version including all levels from Sonic 1, Sonic 2 (including scrapped levels) and Sonic 3 & Knuckles.
Sonic 3 Complete - What can best be described as Sonic & Knuckles Collection on the Sega Genesis, including Sonic 3 & Knuckles, Sonic 3 Part One, Sonic & Knuckles, Blue Spheres and various bug fixes and improvements.
Sonic's Fun and Easy Adventure - A joke hack by Ralakimus which includes the most frustrating moments from Sonic 1, Sonic 2, Sonic CD and Sonic 3 & Knuckles.
Sonic the Very Useful Engine - A parody hack by VadaPega that makes most objects have the face of Thomas the Tank Engine.
Amy Rose in Sonic the Hedgehog - A series of ROM hacks by E-122 Psi that replaces Sonic with Amy Rose, complete with her Sonic Adventure moveset; an alternate version of Amy Rose in Sonic 2 called Sonic 2: Pink Edition replaces Tails with Cream and adds Big the Cat.
Sonic 3D Blast: Director's Cut - A hack of Sonic 3D Blast made by the game's programmer Jon Burton to fix various criticised aspects.
Sonic 2 XL - Another infamous hack that has Sonic eating onion rings, which causes him to get fat until he eventually dies.