We have lots we love (Moodle, Koha, Panopto, etc...) but the only one we use that I haven't seen mentioned below is BigBlueButton (BBB).
We use BBB for video and audio collaboration as well as file publishing/annotation for digital presentations. We've only tested it with 14 participants all sharing video and audio.
BBB is open source, has very low server requirements, and is easy to install/maintain. We've been running it for a couple years with no problems, it has excellent Moodle integration.
There are only two downsides to this tool:
1 - It uses flash instead of HTML5 - so client compatibility is not as solid as we would like to see. This should be addressed in a future version
2 - It uses flash for audio processing instead of WebRTC - This means headsets style mics are essential for participants sharing audio. Unlike Google Hangouts, without the headset style mic terrible audio feedback is heard by all participants. This should also be address in a future version.
Chad, thanks for this. Used to use DimDim before they were bought out by SalesForce and played a little with BBB. Good to see you've had a positive experience.
Not sure if this thread is monitored anymore...but we've had good success with: