The SRB2 Skybase is a rather small community for various things. Originally just a Sonic Robo Blast 2 fan community, it has over time evolved into a community for more than just that.
Sonic Robo Blast 2 (aka SRB2) is a 3D Sonic fan game that uses a heavily modified Doom engine. SRB2 is inspired by the classic Sonic games and is developed by the Sonic Team Jr. fan development team. It has online netplay, a large library of addons, a vast number of zones, many hidden secrets, and more! You can get SRB2 from the official SRB2.org download page, located here.