Trimble SketchUp is a free 3D modeler.

When creating your models for 3D printing your file needs to be one continuous, solid object. The printer must be able to clearly distinguish between the inside and outside of the object it is printing. There cannot be holes or gaps in the model. There are tools that can help identify an close holes and gaps, but the majority of these areas should be found and closed within the original CAD model for best results.