Aside from the 3D Printing Toolbox, Blender 2.67 has another cool feature coming up that'll make precision modeling much easier. Check out this video by Blender 3D Architect. I'm getting REALLY excited about this release :)
Alan Britto writes:
A new feature of Blender will probably help architectural visualization artists that want even more control over precision modeling. A 3d ruler and protractor were added to the latest builds of Blender and will most likely be available on the next stable release (2.67). If you want to check this new feature right away, you can download the latest build of Blender from the Blender BuildBot. Just a warning, those are Blender builds that are not considered stable, so be aware of bugs and potential problems. Do not use them for important projects!