RocketChat

A great, free, open source tool as it evolves rather quickly and provides numerous features. If you know Slack, Discord etc. you should not be deceived by RocketChat.

I use Rocket.Chat as our main chat platform at SQAsquared. We needed a Slack-like communication platform that allowed us to own our data in our data centers. Rocket.Chat enables us to do just that. We've used it for years with no need to switch. Very happy with the product.

Rocket.Chat is a great and easy to use product. We love to use it at Univention ourselves. Thanks to the preconfigured UCS App Appliances, which are available as a virtual image, it is extremely simple to set up and to test it in your IT environment. https://www.univention.de/produkte/univention-app-center/app-katalog/rocketchat/