A Screen Ruler may be a helpful virtual instrument for web designers, as it lets you measure the distance between two points on the screen expressed in pixels. To apply it just click the starting and ending points on the grid. To take the points off, click again. To create a direct line - press “Shift” key. To close the widget right click with your mouse.
Linux users have to use a plainer version that is included here. That is explained by Linux problems with transparency. If you don’t apply Windows or Macintosh this opera widget will find automatically it and switch to the version that supports it.