Don't let the 'fangame' moniker fool you. This is the real deal. The game, called Sonic After the Sequel, is actually a sequel to an earlier game done by the same guy, Sonic Before the Sequel.
Both are full-fledged Sonic games with their own storylines and stages, taking place between the official games: Before the Sequel between Sonic 1 and 2, and After the Sequel between Sonic 2 and 3. They also both feature their own custom soundtracks, and let you play as either Sonic or Tails.
LakeFeperd, the games' developer, is already hard at work creating another sequel, called Sonic Chrono Adventure, which will involve new characters, time travel, and a deeper story.
Sonic After The Sequel Is Now Available For Free [DSOGaming]