Back in 1996, Sonic 3D Blast turned the Sonic world upside down. Providing an isometric camera angle, the Genesis/Saturn title was polarizing in part due to its perspective shift: which is why this fan project sought to reconfigure it to be more in line with the classics.
Cleverly titled "Sonic 3D in 2D," this project from Sotaknuck provides 15 acts across eight zones, as well as classic items, three playable characters (Sonic, Tails, Knuckles), a physics engine that's "close to the Genesis games," and Easter eggs. Sonic can peel out and spin dash as normal, with Tails able to engage in flight and Knuckles climbing/gliding.
As is the case with most fan games, it's free to download. Thankfully, Sega has a more relaxed view on Sonic projects, even hiring fan game developers to work on official Sonic titles.
Although the move to "3D" proved to be somewhat of a wash for Sega in this particular case (and would be many times over in the future), 3D Blast deserves to be preserved: whether that's through its Steam port or the many mega collections it's graced. This fan game also continues its legacy.
Sonic 3D in 2D [Sotaknuck]