This photo gallery of apps is also available as a post in the Five Apps Blog.
First up, MegaMeeting is a cross browser and platform system that allows for one-on-one video chat, multi-party video conferencing, and webinars all within a browser based app for Windows, Mac, and Linux. The app requires only Adobe Flash on these platforms. In addition, MegaMeeting offers iOS and Android apps for connectivity from mobile devices. Attendees are limited by seats purchased. There do not appear to be any free versions; MegaMeeting advertises that their services are as low as $39 per month. Video conferencing is limited to 16 streams but can have hundreds of viewers (no limit was specified).
Screenshot of MegaMeeting app by Wally Bahny for TechRepublic