Visual Studio 2010 has the ability to undock code windows by clicking and dragging on the code window tabs. This is a great feature and it is often touted for multi-monitor setups where you may want to have the form on one window and code on another window, or maybe two different code files open at the same time.
That is very useful, however that is not why I like this feature. I like the ability to undock the windows because it lets me view two different parts of the same file at the same time. Visual Studio 2010 and earlier have supported this using the split view option () but that means you lose half of your screen (like below).
Using the undock windows in 2010 you can use the Windows –> New Window option to duplicate the current window.
Then you can drag the second (or third, or forth) windows out and get a full screen side by side view which is GREAT for comparing code.