X Window Information is a window information utility for developers of
applications, toolkits, and window managers. X Window Information
follows your pointer providing information about the windows below.
Information presented includes:

  *  A detailed description of the window hierarchy below the pointer.

  *  Parsed interpretations of standard properties from both the
     application window and the window manager.

  *  Information about X server resources used by the application.

WWW: http://www.freedesktop.org/Software/wininfo