The protractor tool is very similar to the tape measure tool in that it allows you to add a guideline. This guideline will be at an angle you specify though. This is very handy for lining up items as well as drawing with specific angles.
First activate the protractor too. By default there is no keyboard shortcuts for the protractor tool. Just as with the rotate tool you will be presented with a protractor on the screen that changes color according to the plane in which it is on. If you wish to maintain a certain plane you can first move the mouse so that the plane is represented by the protractor. Then hold shift to maintain this plane and move the protractor wherever you wish to start. The first point selected is the pivot point. Then drag your mouse in the the first direction you wish to measure the angle from. You can select points in the model for reference. Then drag the mouse in the direction you wish to make the angled guideline. Then either click to drop the new guideline or type in the exact angle you wish to use and press enter on the keyboard. You can also select a point in the model as a reference point to drop off the guideline.