OpenSCAD - free software for creating solid 3D CAD models. Useful for creating models of machine parts
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
You can search for pre-existing models on the Internet to print as they are, or to modify using 3D modeling 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)
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)
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