I recently gave the FOSS project known as Sonic Robo Blast (version 2.1) a whirl and I can tell, this isn’t the same low-grade fan project it used to be when I checked on it many years ago.
Looking at YouTube videos, the game really seems to have started to come of age in the latest version (where it brought in the concept of light and shadow as well as better levels and graphics in general). The game’s main render engine is essentially a more powerful version of what powered Doom back in the day (CPU-powered), but it also has a modern OpenGL engine that is being developed (a mode that comes with the download and improves the graphical abilities in some ways, though it is a WIP). You can also check out the code at Github.
What’s more impressive, the project’s been ongoing for 20 years now, and a new version with slopes and more totally redesigned levels (and new ones) is in the works. Now it used to be that this was like Mario platforming with sonic characters, but now it’s more of a hybrid with more emphasis on platforming rather than speed (which is actually what the Genesis games were about too).
If you remember to unzip the archive into its own folder, then it will work out of the box. I’m honestly not sure how many other 3D FOSS game projects (other than 0 AD) have gotten this far and has this quality.