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 focused on video conferencing and offers a range of features for online meetings, while Gajim is a powerful instant messaging client built on the XMPP protocol. Jitsi excels in video and audio capabilities, whereas Gajim provides a robust chat experience with strong security and plugin support.