SketchUp is a free, 3d modeling program which Google bought, then sold to Trimble Buildings. It's designed for architectural, civil, and mechanical engineers.
SketchUp is available for free download from www.sketchup.com
Using Trimble SketchUp
There are lots of tutorials available for SketchUp.
Forum post with excellent usage tips and advice: Re: Sketchup 3d Models – Update 12th Jun 2012.
See also PhlatScript for one technique to convert to G-code.