At the risk of being accused of blowing one's own horn, we have an open source project which implements a Slack-like platform. It's been in production for well

At the risk of being accused of blowing one's own horn, we have an open source project which implements a Slack-like platform. It's been in production for well over a year for us and has worked quite well. We implemented it when we realised that there were no such products available that was open source and self-hosted. Another neat feature (in my opinion, of course) is that it's implemented in Lisp, which makes it much more fun to develop for. Github repository: https://github.com/cicakhq/potato https://github.com/cicakhq/potato Demo server: https://potato.dhsdevelopments.com/ https://potato.dhsdevelopments.com/