Show all windows on hover/click doc icon (Mac Yosemite 10.10.5)
Double tap (not click) the open application icon. This will display all open windows for that application and you can select the one you need.

If you are using a non-Apple mouse/trackpad you can right click or two-finger click an open application icon to reveal a list. The top entries are the open windows for that application. You can then select one of the open windows from that list.