Yesterday in the late afternoon we tested the most recent versions of Dimdim (free version) and Skype (4.1 beta). Each product is making progress. Dimdim is more of a full Web conferencing product, though it seems to me it gives all the control to the meeting owner and very little to the attendees in the session. Further, it had problems with IE (worked fine with Firefox). Audio quality was good but we were all in the office so had a very good network connection.
Skye has screen sharing in its most recent beta version. It is a little confusing because the menu item for screen sharing shows up only when you have a contact selected, but it functions very well when you do youse it. Image is sharp and it can be set to full screen. Right now the downside is that you can only do screen sharing with one other person. So it is still not a tool for small group work. If they do get it to function in a multi-party call, watch out.
Screen sharing in Skype does not allow the user to pass control to others in the call. In that sense regular Web conferencing is more feature rich. But Skype has a huge market base and the product is designed to do what it does very well, which I believe is more important than having additional features.
I can't predict where this market is going and if "free versions" will prevail. But I do like that there seem to be multiple viable players in this area. That's a definite plus.