DimDim is an open source online meeting software solution. You can share your desktop, show slides, collaborate, chat, talk and broadcast via webcam with absolutely no download required for attendees.
The web meeting service is available in open source and commercial enterprise editions capable of supporting thousands of attendees. You can you install it on your servers or they will host it on theirs.
Take a look at DimDim.
If you rather get started immediately, try the GoToMeeting Free Trial