ZBar is a simple tool specifically designed for those users who use multiple monitors at work. It lets you, from a simple screen, manage all of them, letting you see what is on esch one, and move from one to another.
ZBar creates a type of taskbar for each one, that are stored at the bottom of the screen. You just access each one when you want, to see the monitor content.
A basic tool but very practical so as not to go mad with too many monitors.