During the Google Summer of Code 2018 Mentors Summit, many projects used the open-source Zulip Chat for discussing their projects and planning their next steps.
It is a fine little chat in the spirit of Slack, but fully open-source.
We started using it for our projects and for our general discussions. So if you are looking for us, want to onboard the projects or have an idea to propose, we welcome you to join our ZulipChat channel. We are looking to meet you on