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

BigBlueButton supports sharing of slides (PDF and PPT), video, whiteboard, chat, voice (using Freeswitch or Asterisk), and desktops. It's built using over fifteen open source components, runs on Mac, Unix, and PC computers, and is backed by an open source community that cares about good design and a streamlined user experience.

Features
Webcam, Presentation, Integrated VoIP, Desktop Sharing

Demo:
http://demo.bigbluebutton.org/