If your primary criteria is "free" software then you should probably check out Google SketchUp, a free CAD application. There are more full-featured CAD alternatives as well as a paid commercial version of SketchUp, but they will set you back a good deal more than SketchUp and SketchUp is more than adequate for most woodworking purposes.
We have a dedicated forum on NCWW specifically for questions and support of CAD questions. Dave Richards is our resident SketchUp expert. I can't offer support beyond this recommendation as I am a longtime TurboCAD Pro user, so I am not familiar with SketchUp in any great detail.