Sonic on Angel Island
A short, two-level Sonic fangame from sometime in the 2000s. (It's sometimes called Sonic in Angel Island, depending on where you look.) It attempts to emulate the 2D Genesis titles, with the player traversing Angel Island by speeding through loop-de-loops, jumping on robotic enemies, and fighting Dr. Eggman at the end.
This version is said to be 0.5, though whether or not any other versions were ever made is currently a mystery (along with the identity of the game's creator).