I'm very interested in this as well. I've grudgingly been using Blackboard Collaborate for my otherwise free software-driven Wikipedia course.
One thing to look at is Big Blue Button. It's FOSS designed as an educational webinar platform. I have not looked into it extensively, but will report back if I do -- and would welcome others' take on it too! (I believe it requires Flash. Blackboard Collaborate requires Java. I kind of doubt it's possible to escape this kind of requirement at the present time...but I always hope to be proven wrong!)