I use a dual display at work and my primary display is a delicious Viewsonic VA2226w 22-inch widescreen display. The widescreen display is very useful while debugging. But, while editing code most of my source code lies within 100 columns, and the left half of the display is essential blank (and wasted). So, I find it useful to split the Visual Studio edit window into two vertically. I can edit a file and keep an extra file (say header file with class definitions) open on the left to refer to while editing.
Vertical splitting like this is very easy: Choose
Windows → New Vertical Tab Group.
This feature also works in the free Express Editions of Visual Studio.