Alternatives to: Discord

Agorakit is an Open Source groupware for citizens to organize and collaborate online, facilitating communication, scheduling, and information sharing.

Apprise - Push Notifications that work with just about every platform!.

Nextcloud Social is a collaborative app that adds social networking features to Nextcloud, fostering communication and engagement among users.

Open source alternative communication platform

Rocket.Chat is a secure, collaborative communication platform that enables real-time communication and streamlined workflows for teams and organizations.

Zulip is an Open Source team chat application that combines the best of email and chat for enhanced team communication and productivity.