The Finder's "Window" Menu!
All the currently open Finder windows are displayed in the "Window" menu of the Finder. There are three possible indicators at the left of the name of the window: nothing, a checkmark or a diamond. When there is nothing at the left of the name (like next to "Scratch Disk" in the screenshot), it means the window is opened but not currently active. With a checkmark (like next to "Macintosh HD" in the screenshot), it means it is the currently active window. A diamond at the left of the name (like "LaCie" in the screenshot) means the window is minimized.