It doesn't take long in theatre for the great debate between drafting software to rise to the surface. While many will stand behind Vectorworks or AutoCAD (in much the same way that they will PC vrs. Mac), and others will throw in IntelliCAD, Sketchup, and 4 or 5 other programs, I will save that debate for another time.
I am here today to introduce Napkin Cad http://www.napkincad.com/
Portable, fast, easy to distribute, and easy to use. What more can you ask for? You can download a PDF at the above site.