- "Gold Standard" seems to be Slack
- Open-source, self-hosted alternative is Mattermost. Extremely similar UX to Slack.
- Gamer-marketed Discord ticks most of the same boxes, but is designed for more open communities, and lacks video calls (but does voice calls just fine, and will handle screenshare for a limited number of attendees).
- Open-source "slack-with-threads" is Zulip, wants to use a smaller number of channels with consistent team membership, and then create ad-hoc named threads (with their own history) to spin off discussions so that those conversations can be followed easier.