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
open3mod - A free, Open-Source 3D Viewer designed to make it all easier. Massive 3D Format support
cmftStudio - GUI counterpart of cmft - cross-platform open-source cubemap filtering tool.
Cura - Cura aims to be a end solution for personal 3D printing with RepRap based machines.