Much like a screen ruler, I sometimes need to use a protractor on the display to measure some angle. I found this tool named qProtractor that does the job perfectly!
qProtractor has packages for some old versions of Ubuntu. For my version of Ubuntu, I had to download the source and compile using
make which was successful.
On running qProtractor, it displays a full 360 degree protractor which remains always on top. You can move it to anywhere on the display. Right-click to change properties. A few things you can do is to rotate it so that the zero degrees begins whereever you want and to scale the protractor up and down to fit your needs.
Tried with: QProtractor 0.0.1 and Ubuntu 14.04