MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.
FreeCAD - An open-source parametric 3D modeler
Slic3r - STL-to-GCODE translator for RepRap printers Here comes Slic3r! Its features are: - speed
SketchUp - 3D for Everyone
Cura - Cura aims to be a end solution for personal 3D printing with RepRap based machines.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.