HyperDock is currently in free beta and allows Mac users to add Windows 7 taskbar style thumbnails to the Mac OS X dock, as shown in the image above.
One of the most useful UI improvements in Windows 7 is the taskbar, specifically the thumbnails of open application windows. This is a very practical method of quickly accessing a specific window from an application, which currently has several windows open. Of course there is the Expose´ feature on OS X for managing application windows, but many find it awkward to use and not as fluid as the thumbnail functionality with the Windows 7 taskbar.
Other than being able to see thumbnails, there are other useful features such as being able to control iTunes and view iCal events from within thumbnails.
You can download the free HyperDock beta now for Mac OS X 10.6 or higher.