I wonder if mid-2014, with a massive rise of WebRTC, are there any free tools allowing video conferencing or just audio chat for 15/20/50/100 participants?
Skype: http://www.skype.com/en/group-video-call/ (up to 10)
Google Hangouts: https://webapps.stackexchange.com/questions/53098/how-many-people-can-be-on-a-google-hangout (up to 10, up to 15 for biz / edu / gov)
GoToMeeting: http://www.gotomeeting.co.uk/meeting/pricing (up to 25 in basic paid version)