Autodesk 123D Design is a free, powerful, easy and user-friendly 3D application that can create amazing and complex objects starting from basic shapes or sketches.
The objects can be 3D printed or manufactured with CNC, laser cutters, water jet cutters, etc.
SketchUp – formerly Google SketchUp – is a full-blown 3D modeling software that is now owned by architectural company Trimble Buildings. By installing different plugins, you can adjust SketchUp up to your personal preference. SketchUp is available as a commercial version called SketchUp Pro and as the free SketchUp Make edition.
Blender is great free program to either create models for printing, games or video clips. It contains many useful tools for example various algorithms for smoothening or interpreting a surface.
Blender also requires a powerful PC to display some models, especially if you subdivide the model into many smaller tiles.
FreeCAD is an open source and free to use CAD software. It offers many different tools for construction, like Drafts, Drawings, and Design features.
A serious limitation:
FreeCAD has some problems with intercepting structures and may mess up the mesh when it contains intercepting edges!
MeshLab is a free open source program that allows you to view or merge, transform or repair STL, PLY, STL, OFF, OBJ, 3DS and many other file types as well as point clouds.