Blender - Blender is the open source, cross platform suite of tools for 3D creation.
LeoCAD - LeoCAD is a CAD program for creating virtual LEGO models.
FreeCAD - An open-source parametric 3D modeler
LDraw - LDraw is an open standard for LEGO CAD programs that allow the user to create virtual LEGO models...
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.
Stud.io - Build LEGO designs online with friends.