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
Repetier - Software for controlling RepRap style 3D-printer like Mendel, Darwin or Prusa mendel.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.