BigBlueButton (http://www.bigbluebutton.org) is an open source project that is built on over fourteen open source components to create an integrated web conferencing system that runs on mac, unix, or PC computers.
It supports the following capabilities:
Web Cam. Multiple users can share their Web cam at the same time. There is no built-in limit on the number of simultaneously active webcams.
Presentation. The presenter can upload any PDF presentation, office documents and keep everyone in sync with their current page, zoom, pan, and see the presenters mouse pointer.
Integrated VoIP. BigBlueButton voice conferencing supports voice over IP (VoIP) conferencing out-of-the-box. All your students need are speakers and a microphone to participate.
Desktop Sharing. Go beyond slides. The presenter can broadcast their desktop for all students to see. Works on mac, Unix, and PC.
A slide-cast for this plug-in is available at www.slide-casts.com