About three months ago, I went dual screen. That is, I switched from a single computer monitor to a dual screen monitor.
I had read that a dual screen monitor can increase productivity by 50%. While I don't believe this statistic, the dual screen monitor definitely saves me 10 - 15 minutes/day.
The dual screen is really helpful if I want to comment on website, for instance. I'm able to have my email open in one window and my Internet browser in the other, so I don't need to constantly toggle back and forth between screens.