There are three programs for whiteboard/desktop sharing.
Microsofts NetMeeting for windows. Irix's SGIforum, and the X windows program SunForum. They use the T.126 protocol. Googling for 'T.126' in quotes may provide more answers for you. (Which is how I found out the information on these programs)
I think that this is the standard way of doing what you want to do.
These three programs should be inter-operable, so you can use SunMeeting to conference with someone using a NetMeetiing3 for example.
Here is the URL for the SunForum datasheet: http://www.sun.com/desktop/products/.../datasheet.pdf
However I don't know of any realtime Desktop -> Webpage programs that you seem to be hinting of.