This tool is designed and made for one single purpose - to help you to measure anything on your screen. The tool is highly configurable, comes with different useful options:
* measure in pixels, millimeters and centimeters
* horizontal and vertical guides
* customizable translucency
* customizable tickers
* "Stay on TOP" window style (floating over other windows)
* Screen edges snapping
* Remembers last state (position and options)
* Mouse and keyboard moving/resizing
* FREE to use

The user-interface should be friendly, but who knows...

* The ruler is transparent and you can customize the level of transparency

* Use the left mouse button to move the ruler - as usual for all windows programs

* To use vertical and horizontal guides - left click over the respective guide

* To close, change units etc. -- right click over the ruler.

* The program comes with system-notification area icon - right-click over the screen ruler icon to open the menu. To put screen-ruler in the notification area (system tray) just click "minimize to tray"...