Is there any free webinar plugin available that is open source and free to integrate and use?
Hi,
Try BigBlueButton or OpenMeetings both have activity modules for Moodle.
http://code.google.com/p/openmeetings/
Regards
Alex Contis
NetSapiensis AB
Both Big Blue Button and Openmeeting have Moodle plugins and are Open Source.
You will need to build the servers yourself. A relatively cheap commercial conference server is Wiziq also with Moodle plugins.