MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.
open3mod - A free, Open-Source 3D Viewer designed to make it all easier. Massive 3D Format support
Slic3r - STL-to-GCODE translator for RepRap printers Here comes Slic3r! Its features are: - speed
CtrlView - Fast, compact, powerful viewer/converter for different 2D/3D raster/vector file formats
Cura - Cura aims to be a end solution for personal 3D printing with RepRap based machines.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.