Google SketchUp Pro for Mac

From Google:

Developed for the conceptual stages of design, SketchUp is powerful yet easy-to-learn 3D software. We think of it as "the pencil of digital design". This award-winning product combines a simple yet robust tool set that streamlines and simplifies 3D design. SketchUp is used by professionals and consumers alike. It contains a unique interface that allows 3D forms to be created, viewed, and modified quickly and easily. The core of its simplicity is that you draw the edges of the desired model in 3D space, just as you would using a pencil and paper. SketchUp infers your design intent and automatically determines the nature of the lines and fill shapes to create 3D geometry.