In the early days at my company, we had an arrangement where the CEO had 3 monitors on his desk. Two faced him, and one faced the other direction and mirrored one of his screens. There was a second keyboard and mouse attached so that someone could sit on the other side of the desk and collaborate. He could keep private data like email on one monitor and other applications on the shared screens. Somewhat comically these were enormous 21" CRT's so by "desk" I actually mean "dining table"
I really enjoyed working this way and kind of wish the same experience could be replicated between multiple machines and with more than 2 people. I would like it if anyone could drag an application onto a shared screen where multiple people could control/interact with it while still having the option to take a window from the shared display back to a private display, ie passsing an application from one system to another.