MB-Ruler installs a virtual set-square on your desktop that allows you to measure distances and angles, among other functions. This set-square is almost transparent with the intention of not interfering your work with other apps, though this is not always possible. The program allows you to define the measuring scales and units. You can select units among cm, mm, pixels, inches, points, and twips.
MB-Ruler, however, may be hard to understand at first. It installs two icons on your system tray area, one for the MB-Ruler itself and another one for the "cut screenshot" tool, which allows you to capture screenshots. You need to right-click on the MB-Ruler icon to display its main menu, from where you can select the measuring instrument, set the scale and units, set the grid and bitmap overlay options, and open the settings window, among other tasks. One of the new features offered by this version of the program is that it handles user-defined measure instruments. Besides, the program includes some additional tools, like a magnifying glass and a RGB color picker.
As I stated before, despite MB-Ruler was designed to be transparent, it may become annoying after you have been using it for a while. Therefore, I would recommend you to run it only when you really need to measure distances or angles, or when you need to use some of its additional tools. The program is compatible with all MS Windows versions available at the moment, and its system resources consumption is fairly low.
In conclusion, if you are a cartographer, a geologist, an analytical geometry teacher or any other professional requiring to measure distances or angles, I would definitely recommend you to give MB-Ruler a try, though it may take you a while to get used to the program's use and behavior.