Download Chromolys Ruler: versatile ruler/measurements tool



First go to User Preferences->Addons and enable the Ruler addon in the 3D View category.
Enable and Ruler shows up in the 3Dview properties panel.     Click enable to enable,
click disable to stop.
Use Ctrl-Shift-R shortcut to switch to measurements mode and back.
When in measurements mode press "H" to get a list of possible key commands.
More docu soon...