A few days ago, I upgraded to 3 screens at my day job office. I did this mainly because I found myself switching too much between my laptop screen and the external monitor. One more monitor, it seemed to me, would help make things more efficient.
From left-to-right, I use the (new) large 24″ monitor for my active development, usually in SQL Management Studio or Microsoft Visual Studio 10. The center (17″) monitor is used to display results. So I can compile and/or debug in the large monitor and see the results in browser windows in the center monitor. The laptop screen is used for email and jotting notes in Evernote.
Incidentally, the version of the Dell laptop that I have does not support 3 monitors. I had to get a VGA USB display adapter in order to drive all three screens at the same time. Still, once I plugged in the adapter and connected the monitor to it, everything worked, and now, I have it setup just the way I want it.