When it comes to open source videoconferencing, the proprietary world has been leading the charge with solutions like Zoom, Google Meet, Microsoft Teams/Skype. Since the issues with Zoom on privacy and data protection, and the Cornovirus crisis, the need for open and secure video conferencing raised significantly.
The leading open-source solution is Jitsi (formerly SIP Communicator), an audio/video conferencing and chat communicator software that supports protocols such as SIP, XMPP/Jabber and many other useful features. Jitsi is Open Source / Free Software, and is available under the terms of the LGPL. Jitsi Meet is the most advanced version with encryption and client app for iOS and Android. You can host is on your own server or get a full cloud version like Zoom.
BigBlueButton is another alternative. Feature-rich, it is not that easy to install but provides a rich user experience. Heavy on resources, you could look at a hosted solution.
Linphone is also an enterprise solution with double license libre (opensource) and commercial for video/ voice calls. It is basically the ideal alternative for Skype on this list. Linphone has native mobile apps for Android, iOS, and desktop clients for Windows, Mac OSX, and Linux.
Tox is something you could look at if privacy is your #1 need. Built to avoid digital surveillance, it is still young but with a growing community.
OTHER OPEN SOURCE VIDEO CONFERENCING SOFTWARE
VMukti is Social, Web 2.0, SaaS, Peer-to-peer, & VoIP Open Source multi-Party Conferencing (like video and web conferencing WebEx) Broadcasting , Collaboration And CMS platform for Web & Mobile users Video/ Audio/ Presence & Content.
Openmeetings is a free open source video and web conferencing software. You can use your microphone or webcam, share documents on a whiteboard, share your screen or record meetings. It is available as a hosted service or you download and install a package on your server with no limitations in usage or users.
Ekiga (formerly known as GnomeMeeting) is an open source SoftPhone, Video Conferencing and Instant Messenger application over the Internet.
It supports HD sound quality and video up to DVD size and quality.
It is interoperable with many other standards compliant software, hardware and service providers as it uses both the major telephony standards (SIP and H.323)