There are many FREE CAD suites that make designing for the 3D printer a breeze. Each one has its strengths, so try 'em all and see which works best for you!

• Blender: This feature rich software can create everything from solid models for printing to full fledged, cinema quality, video. Although the learning curve is a bit steep, there is a huge amount of support available in their community forum and elsewhere online.
• Tinkercad: Revived by Autodesk, Tinkercad provides an easy to learn solid modeling environment that links directly to your Thingiverse account.