Screen Ruler, which can be found in your Ubuntu Software Center is great.
But there is also another excellent tool that is available from installing Screenlets from the software center. Just type "ruler" in the search box after installing screenlets.
Well if you are going to use it for web design or web development then I would use the Web Developer extension within the FireFox Web browser. It also has tons of other useful tools which can be very helpful in your designs or development. It works great for CSS designs.
There are two packages in the repositories:
Screen ruler
sudo apt-get install screenruler
kruler
I haven't tried either of them, but they might be what you're looking for.
Screen Ruler, which can be found in your Ubuntu Software Center is great. But there is also another excellent tool that is available from installing Screenlets from the software center. Just type "ruler" in the search box after installing screenlets.
Well if you are going to use it for web design or web development then I would use the Web Developer extension within the FireFox Web browser. It also has tons of other useful tools which can be very helpful in your designs or development. It works great for CSS designs.
Take a look at it here https://addons.mozilla.org/en-US/firefox/addon/60/ . I also think they have it for Chrome. This will provide you with a great on screen ruler for all your design projects.