For those who don't know, SS13 is an open-source multiplayer round-based roleplaying game that has been built on the closed-source BYOND engine since 2005. Imagine Among Us, DarkRP (on Garry's Mod) or NoPixel (on GTA V) but with much more content for both antagonists and normal workers. Or Dwarf Fortress/RimWorld but everybody controls a different person (with usual crew count being 50-150 people).
It has a great number of forks, with more than a dozen focused on the space station gameplay loop and various others that adapt the game to themes such as marines vs xenomorphs missions, an open-world modern day city setting, ancient civilization world, etc.
Other than that if you want to find out more I'd suggest watching the two most popular game reviews about it I added in another comment https://news.ycombinator.com/item?id=29109264