open3mod - A free, Open-Source 3D Viewer designed to make it all easier. Massive 3D Format support
Cura - Cura aims to be a end solution for personal 3D printing with RepRap based machines.
CtrlView - Fast, compact, powerful viewer/converter for different 2D/3D raster/vector file formats
Slic3r - STL-to-GCODE translator for RepRap printers Here comes Slic3r! Its features are: - speed
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Repetier - Software for controlling RepRap style 3D-printer like Mendel, Darwin or Prusa mendel.