Many a times when you design websites or application, measuring the dimensions is necessary to develop a usable user interface ( UI ). Nruler is one such application which can help you measure in pixel right from your desktop.
It creates 2 virtual ruler one of which measures vertically and one which measures horizontally in pixels. The rulers can be expanded depending on your screen resolution which helps you testing even further on different resolutions. A very handy application which can be downloaded for free from here.