I am a Ustreamer. I used it this summer at an international convention in San Antonio. We had over 200 viewers watching at one time from all over the world. It also has a chat room that viewers can interact with each other as well as the producers of the show.
As a producer, you have the ability to adjust the video and audio quality. It is also archived for later purposes.
I also use a Web2.0 program called Mogulus.
http://www.mogulus.com/
Mogulus is a complete studio control system. It allows you to have multiple cameras running at one time. It is a free system just like Ustream. You can prerecord and play video or screenshots as well.
Hope this helps.