I gonna test the features.
Blender 2.66 r55116 3D Ruler & Protractor Tutorial
First ruler: Click with the LMB on the first place and drag to
the second place.
Ctrl + LMB to create a new ruler.
Ctrl while drag for snapping.
LMB on the middle of the ruler to change it to angle ruler.
Erase the ruler with Delete key
Esc to exit ruler mode.
Blender Architecture Measurments Tutorial!