I've had a private Matrix server for me and a few family members for years now. It doesn't federate, and I'm afraid to try, because I run it on a potato computer and I've heard horror stories of Synapse federation consuming hundreds of gigabytes of space.
It's generally been stable and performant and really nice to use. The only problem is encryption: Element seems to forget keys sometimes and need to resync. And Matrix's whole e2ee system is arcane nonsense to anyone who's not extremely technical. Every time that happens, it's a long tech support call.
The solution is to just not use encryption on chatrooms, which is fine, but DMs have to be encrypted so it still comes up sometimes.
I'm surprised hardly anyone else has mentioned Wesnoth yet. I last played it over a decade ago, so I'm not sure if it's still as well-known now as it used to be, but it's one of the highest-quality open-source games out there.
It's a turn-based fantasy strategy game that feels like a combination of Advance Wars, Fire Emblem, and D&D. And I suppose it was a lot more unique back in the 2000's when there weren't a dozen indie games out there that fit the same description.