While looking for open / free collaboration tools, I found DimDim. From their website:
"Dimdim is a free web conferencing service where you can share your desktop, show slides, collaborate, chat, talk and broadcast via webcam with absolutely no download required to host or attend meetings."
Here is a good demo of how it all works.
(And here is an open source version of their product. Signing up is free.)
I recently tried it with Linux, and screensharing is indeed a problem. Everything else seems to work ok, though. (More here)