MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Slic3r - STL-to-GCODE translator for RepRap printers Here comes Slic3r! Its features are: - speed
FreeCAD - An open-source parametric 3D modeler
Cura - Cura aims to be a end solution for personal 3D printing with RepRap based machines.
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.