This is Sonic Robo Blast 2 v2.2.
This Sonic fan game has been around since 1998. Despite the age, the developers are still working to this game even to this day. Yeah, you heard me right.
It's not like this game is incomplete, it's just that the developers seems to have fun adding new features and tweaking the game.
This game also has a small but super passionate fan base. Since the engine is super easy to program with, the fans of this game created A LOT of mods.
Oh right, forgot to tell you that this fan game has been created using the Doom engine... Yeah, Doom!
Between characters mod and new maps, this game has a lot of user-created content to offer. The most popular one is the Sonic Robo Blast 2: Kart... Basically a Sonic themed Mario Kart game.
A week ago, they released a new update for the game: version 2.2.
This is a huge update that added a lot of new content and overall improvements to the base game. Stuff like new high quality sprites, improved textures and graphics, new and/or extremely tweaked levels, new playable characters, new original music... and slopes!
If you're interested in a rock solid 3D Sonic game (even if it was made with the Doom engine, so it's technically not 3D) with calssic era games aesthetics and vibes with a lot of content and multiplayer functionalities, you might want to check this out!
If you're not interested, well... I suggest you to try it anyway! It's a really good game and it's a freeware, mate! You can play it for free!!