BigBlueButton is an open source web conferencing system for distance education.
It supports sharing of slides (PDF and any document readable by OpenOffice), webcams, whiteboard, chat, voice over IP (using FreeSWITCH), and presenter's desktop. It can record and playback sessions (slides, audio, and chat), runs on Mac, Unix, and PC computers, and is supported by a [community of developers] (http://www.bigbluebutton.org/support/) that care about good design and a streamlined user experience.
After watching the videos, if you want to immediately try out BigBlueButton, the project maintains a live [demo server] (http://demo.bigbluebutton.org) that anyone can join.
If you like the work we've done with BigBlueButton and would like to contribute back, see [Contribute to BigBlueButton] (http://code.google.com/p/bigbluebutton/wiki/FAQ#Contributing_to_BigBlueButton).