Request for sources: Discord alternatives

I'm writing up a survey post on "chat systems you might actually want to use besides Discord", and it seems like every time I talk about this someone chirps up and asks about something new I've never heard of before. So, I want to try to head this off at the pass and collect as much as I can right off the bat. So, anyone have anything they want me to cover? Things on the list so far: * Mumble * Zulip * Stoat * Signal * Matrix * XMPP servers -- Prosody, Snikket * XMPP clients -- Dino, conversations.im, snikket, mov.im * IRCv3 * Root * Polyproto * Spacebar * Rocket.chat * Mattermost * Tailchat