Have alread uses:
- openProject (though not that much since it was a one time occurance but the software was absolutely great!)
- xwiki: since a few months (work migrating away from the atlassian clusterfuck). Migration a bit of work but a paid tool did the most part for us. Now using xwiki - most things work the same. Some though had to be realised in a different way (some rare edgecass confluence plugins that were missing in the xwiki store)
- Mattermost. Using it for years. Its fucking great! Easily the best one on the market! Better than teams and whatnot.
I'll look into the rest of those alternatives! :D