Starters for ten:
1) Minecraft / MineTest server 2) IRC server 3) Mastodon server 4) BBS 5) ... please continue
I'm not fully sure about all this - as I understand it, if you allow chat or user generated content, you become a private service provider, and then have a host of responsibilities including annual audits of whether a significant proportion of your users are minors. If they are then you need to use a (commercial?) age verification tool and monitor everything assiduously. Difficult to see most people being able to satisfy those requirements.