Sonic the Hedgehog
Sonic the Hedgehog has been extensively hacked.
SonED and Esrael Sonic Editor II are the primary Sonic the Hedgehog editors. SonED works with Sonic the Hedgehog 1, 2, and 3. Esrael Sonic Editor works with Sonic 1, Sonic 2 Beta, Sonic 2, Sonic 3 and Sonic 3K.
There are many Sonic hacks spread all across the internet. For now, refer to the External Links to find good Sonic Hacks.