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.