Mattermost is an open source, self-hosted, private cloud messaging solution compared to Slack which does not have the option to self-host. Security concerns still remain the number one barrier to a cloud computing deployment. With Mattermost, compliance is easy as the data does not leave the customer's premises. It’s perfect for businesses that want to keep their messaging app under tight control. Slack offers 2000+ integrations which covers all applications businesses use. While Mattermost has 100s of integrations as well, the use-case coverage is still limited when compared to Slack.
Zulip is a powerful, open source group chat application. Zulip provides the benefits of real-time chat, while also supporting asynchronous communication. Zulip is inspired by email’s threading model: every channel message has a topic, just like every email has a subject.
Element is an open-source instant messaging client. Element supports end-to-end encryption, groups and sharing of files between users. With Element, conversations are hosted and owned entirely by the participating servers, instead of being stored centrally somewhere in the US, which is particularly attractive to European governments and military.