Google SketchUp Free for You

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 software combines a simple, yet robust tool-set that streamlines and simplifies 3D design inside your computer. SketchUp is being used by anyone with the desire to.

Simple enough.

Link:
http://www.sketchup.com/