We've used MeetingBurner with success for our webinars. The price point for a large audience is not too high. It also has recording and automated webinar capabilities.

I just listened to Marshall Goldsmith and Joseph Grenny on Telenect. It was a great interactive experience. Split screens with a lot of choices...telenect.com

There are 3 that could work: udemy.com, usefedora.com or zenler.com

Rabb.it is also an option