Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Cinema 4D - Cinema 4D is a 3D modeling, animation, motion graphics and rendering application.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.