Does anyone know a good web based meeting solution that allows for live screencasting to meeting participants?
We don't mind installing software on the local machines but the views of the meeting don't have that flexibility so they would require a browser based solution
Webex works well, I've used it fairly extensively on both OS X and Windows extensively at work, and it meets your requirements. I've also used GoToMeeting in the past (note: GTM now apparently supports Mac clients, which wasn't the case when I used it last, so either Webex or GTM can be used for at least Windows or OS X). If you're looking for a free/open source alternative, check out DimDim - I haven't used that one personally but it supports the same basic features and looks interesting enough that I'd like to try it out.
Oh, and one more - I've also extensively used Cisco's MeetingPlace but based on those experiences I would not recommend it. It's much clunkier in my experience and not as smooth an experience as Webex or GoToMeeting.
Acrobat Connect (awkward name aside) is a great piece of web conference software. Runs through Flash Player, so there's nothing to install aside from that. I've used it for years and haven't found anything that works as well or as simply as that.
Microsoft Live Meeting. There is a browser component for viewers but it is not as fully featured as the software install. You can buy third-party hosting for MLM.