FreeCAD - An open-source parametric 3D modeler
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.
Onshape - Onshape is the first full-cloud 3D CAD system. It runs in a web browser and on any mobile device.
Sculptris - Sculptris: Enter a world of digital art without barriers.
Slic3r - STL-to-GCODE translator for RepRap printers Here comes Slic3r! Its features are: - speed