Mattermost is an open source Slack alternative. This is slightly old news but I did find this article interesting because of its emphasis on the open source angle, which I find relevant with the influential open source community.
The battle for team chat in the workplace is heating up just as Slack gets ready for its IPO. Few people thought Microsoft could build a great alternative to Slack 2.5 years ago. Google is also improving Hangouts Chat with a Gmail integration. For another perspective, make sure to also read "Why has Microsoft Teams overtaken Slack with 13 million daily users?" by Jason Lemkin.