Screen sharing applications have been around for years, in the past, we have used GoToMeeting for indepth screen sharing sessions. Vyew offers a similar, scaled back and free service - the biggest difference being that you can't share your screen in real-time, this is replaced by an easy screengrab feature, which pulls the screenshot back into Vyew and allows all viewers to use it as the base of a virtual whiteboard. During my quick demo, I took a few screen shots and played with a few features, and it is kind of slow… But it works. 

 Vyew is purely web based, so no need to download anything, it's free and it enables group collaboration (I think of it as an addition to a teleconference really as opposed to a full on screen sharing session). Vyew's comparison chart gives you a better indication of it's functionality - although it does overemphasise the importance of screenshots over real time screen sharing. 

Vyew seems to be most useful for those who hold teleconferences that don't revolve around sharing screens, but would be aided by a little visual collaboration.