Gimp - www.gimp.org/

Blender - www.blender.org/download/get-blender/

Open office - download.openoffice.org/

Inkscape - www.inkscape.org

Google Sketch up - sketchup.google.com/ with stl plugin

This plugin seems to work quite well  www.guitar-list.com/download-software/convert-sketchup-skp-files-dxf-or-stl

However this program repairs any incomplete stl files www.netfabb.com/

Or this program http://meshlab.sourceforge.net/

Adobe - www.adobe.com

Qcad - www.qcad.org/qcad.html

Eagle - www.cadsoftusa.com/freeware.htm

MIT Software - http://fab.cba.mit.edu/content/tools/software/cad_cam/