Previously, I would create a unit size plane or box geometry and extrude to create a white-box room or structure. This was extremely tedious and caused me much pain. Using this script, I can project all of the faces after I create the room. I just need to be careful to slice the object back into the proper units. This is such a huge time saver.
This script is meant for white box items in line with the grid. It won't really work if any of the projections occur at an angle. Stay tuned for the update, I'll probably tweet it out. Let me know if there are any bugs. Be sure to freeze transformations before and delete the history after.
Download the script on my Github.