If you’re like me, and can’t draw a straight line much less an accurate 3D representation of a structure, OpenSCAD is the cad software for you. Rather than require a patient and steady hand like most CADs all you need is a bit of scripting and walla 3D model.
P.s. For those who like drawing and not scripting, there is always Blender.
Update: In related CAD news, NASA released into the open a pretty sweet modeling tool for aircraft.