LeoCAD - LeoCAD is a CAD program for creating virtual LEGO models.
SketchUp - 3D for Everyone
Stud.io - Build LEGO designs online with friends.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
LDraw - LDraw is an open standard for LEGO CAD programs that allow the user to create virtual LEGO models...
FreeCAD - An open-source parametric 3D modeler