3D Printing

Free 3D modeling software

• Blender - open source 3D animation suite. Enable the 3D Printing Toolbox.
• OpenSCAD - free software for creating solid 3D CAD models. Useful for creating models of machine parts.
• Sketchup - comes in free or pro versions. Get the Sketchup STL from the Extension Warehouse. 
• Tinkercad - a browser-based 3D design platform, now part of Autodesk (free version available). 
• List of additional free software packages from 3ders.com.

Free 3D models

You can search for pre-existing models on the Internet to print as they are, or to modify using 3D modeling software.

• 3D Warehouse - Sketchup's searchable design library. 
• Instructables - from the 123D Design community.
• Thingiverse - MakerBot's searchable design library community.

3D modeling tutorials

• 3D modeling for beginners (Shapeways)
• How do I make a solid model (Rhino)
• How to fix and repair your 3D files (Shapeways)
• Tips for designing 3D printed parts (UT Austin Innovation Station)