<p>I am looking for open source or free data collaboration software. Specifically this is for a non-profit organization that wants to teach remote students how a foreign language.  The idea is that an instructor would teach a class and there would be up to 10 students in the class at a time.  The instructor would be able to post slides or other teaching material and the students would be able to see it on their computers remotely.  Video is not required but audio is a must.  Any recommendations?</p>

<p>Also if there have been any reviews or feature comparison amongst these products, I would be interested in hearing about them.</p>

I know a few of the developers on the Carleton University developed
Blindside Project
.  They are actively developing an open-source web conferencing and presentation tool for e-learning, with the intent of eventually offering university courses online.
It's pretty fully featured software, and is meant to be installed as a server that can host many conference rooms at a time.  It has voice, video, text, and a whiteboard/slideshow (
Edit:
supports PDF at the moment) capability.  One feature I think it neat is that students can 'raise their hands' in the class to ask the instructor a question, where they can take the floor for a moment.
Check out the demo on the site (if it's not working anymore I'll nudge the developers).  Another pro is that the clients only need to have flash installed.
I just logged onto the online demo and created this preview:
This project is now called BigBlueButton :
http://code.google.com/p/bigbluebutton/
Here is the demo:
http://demo.bigbluebutton.org/