Screen Ruler

A screen ruler is indispensable for a programmer who works with images, windows or the web. It is useful to check pixel distances on the display and to check if artifacts in photos are perfectly horizontal or vertical.

Ubuntu

The Screen Ruler tool on Ubuntu does this job perfectly.

It can be installed easily from Ubuntu repositories:

$ sudo apt-get install screenruler

It can be started from the Dash as Screen Ruler. Note that this application uses Ruby and will install that.

Tried with: Screen Ruler 0.9.6 and Ubuntu 12.04 LTS

KDE

KDE Screen Ruler is the best screen ruler I have used. It can show the ruler in vertical or horizontal orientations, ruler opacity is configurable and it has a RGB color picker value that is always displayed on it.

Installing it is easy:

$ sudo apt install kruler

Note that if you do not have any KDE application installed, installing this one may pull in many KDE packages to be installed. This is not a problem for me, since I do use some KDE applications.

Tried with: KDE Screen Ruler 4.13.0 and Ubuntu 14.04

Windows

There are too many shady tools for Windows, I prefer the open-source ScreenRuler by BlueGrams. Get the installer from its SourceForge link.

BlueGrams also happens to have some cool Windows tools like a periodic table, Youtube video downloader, mouse position tracker and keep windows on top tool.

Tried with: ScreenRuler v0.10.0