I have dual video displays on my computers, both home and at work.  For people who constantly switch between two or more applications, it increases productivity by 9 to 50%.  Those aren’t my numbers; they come directly from Microsoft:

Give someone a second monitor, let them use it for while, and then try to take it away. It just isn’t going to happen. They’ll never go back to a mono display. Researchers in the Visualization and Interaction for Business and Entertainment group (VIBE), found that increasing a computer user’s display space made it easier for them to complete their tasks.

[…]

The research study required users to complete several different tasks, switch from one task to another, and remember data. None of the study participants had used multiple monitors before.

The first study revealed that the users’ productivity increased by 9 percent. Further studies showed even greater increases – at times up to 50 percent for tasks such as cutting and pasting. Mary Czerwinski, the VIBE research manager, is excited about her group’s discoveries, asking, “If you’re able to squeeze 10 percent more productivity out, do you know how much money that will save?”

My own observation is that for programming tasks, where the user has an IDE open in one window and the browser open in the other, 50% is a conservative estimate.