BigBlueButton is an open source web video conferencing system developed primarily for distance education.

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BigBlueButton supports multiple audio and video sharing presentations with extended whiteboard capabilities - such as a pointer, zooming and drawing - public and private chat, desktop sharing, integrated VoIP using FreeSWITCH, and support for presentation of PDF documents and Microsoft Office documents. Moreover, users may enter the conference in one of two roles: viewer or moderator or students and professor.

As a viewer, a user may join the voice conference, share their webcam, raise their hand, and chat with others. As a moderator, a user may mute/unmute others, eject any user from the session, and make any user the current presenter. The presenter may upload slides and control the presentation.

BBB features
 
Record and Playback
BigBlueButton can record your lectures for later playback by students.
 
Whiteboard
Whiteboard controls let you annotate and call out key parts of your presentation for viewers.
 
Desktop Sharing
Go beyond slides. The presenter can broadcast their desktop for all students to see. Works on Mac, Unix and PC.
 
Integrated VoIP
BigBlueButton voice conferencing supports voice over IP (VOIP) conferencing out-of-the-box. All your students require are speakers and a microphone to participate.
 
Presentation
The presenter can upload any PDF presentation or Microsoft office document to present, zoom, pan, and keep students in sync.
 
Web Cam
Multiple users can share their webcam at the same time. There is no built-in limit on the number of simultaneously active webcams.