It is sometimes useful to have an overview of the applications that are open in the Dock, like the style of Windows 7. To do so, a tool like DockView is necessary.
DockView allows you to overview the open application in Dock. Once it is installed, you only have to drag the mouse over every icon on the Dock to preview. It includes features for displaying or not some icons, personalizing the Dock. It is easy to handle and its GUI is well designed.