I've recently attended an online workshop using Citrix GoToWebinar, a service that allows someone to broadcast their desktop and audio out to all members connected to the presentation as well as receive audio and screencasts from those attendees , and the experience was unbelievable, and I was wondering if there was any such service/software available on Ubuntu. Points to consider include, but are not limited to:

• Web services or local applications.
• Free (as in beer) or paid.
• Free (as in speech) or proprietary.
• Ease of use
• Ease of setup
• Any other point that anyone can thing of