3D modeling resources

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)

123D Design - a free, powerful, yet simple 3D creation and editing tool from Autodesk

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 

Instructables 

Thingiverse 

CG Trader

MyMiniFactory

Yeggi

Free Slicer Software

Slicer software prepares files for printing by converting your file into machine-readable code (usually .gcode or .zcode). This software can allow you to estimate the amount of time and material a print will consume.

LulzBot Cura (compatible with the Lulzbot TAZ 5)

Slic3r

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)

Other resources for 3D modeling and printing (3D4U)

Other resources

Make Printable - A tool for fixing prints

3D Printing Price Check - Calculate how much your print would cost at a variety of pay-for-print services

3D Insider’s 3D Printing Guide 

3D Printing STL files: A step-by-step guide

3D Printing geometry restrictions

How to optimize your design for FDM 3D Printing