Let me think...
- Flohmarkt is like Craigslist or eBay
- Honk - Ultra-ultra minimalist
- Vocata - a general C2S-enabled server that allows you to throw any kind of Vocabulary you want at it. Could be useful for mocking up client apps.
- Wordforge - federated novel-writing
- SkoHub - Some kind of federated knowledge discovery system?
- GreatApe - an OBS-like federated video thing that you can have live audiences with.
That's just what I could find from scrounging around, I know there's more.
It's actually not too bad to run, it's just that my community instance has grown a lot, and is close to four years old at this point. Issues crop up, I mostly wrote this guide to share some insights on how I deal with things.