Jitsi (formerly SIP Communicator) is a free and open source multiplatform voice (VoIP), videoconferencing and instant messaging application. Developed by University of Strasbourg (France), plus many others.
License: Open Source
Jitsi is primarily a video conferencing tool with a strong focus on high-quality video and audio calls, while Mattermost excels in team collaboration with features like file sharing and chat. Jitsi is best for quick virtual meetings, whereas Mattermost is better suited for ongoing team communication and project management.