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!
- 123D Design: Autodesk's 123D Design provides a very capable solid modeling environment that operates on Mac and PC. The 123D tool set is similar to commercial suites making it a good free alternative to software such as Solidworks and Fusion 360.
- 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. Unlike 123D Design, Thinkercad is capable of importing and manipulating existing solid models and runs entirely "in the cloud."