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What computer software is used to create CAD drawings?

Specifically, two-dimensional CAD drawings that can be drawn to scale. Below is an example of one (click on the download link to view the plans): http://www.littlemachineshop.com/Projects/OscillatingEngine.php I already know that this is a PDF file, but what is the name of the software used to create these plans? Also, what are some more software programs that allow the user to design plans like these and are any of them offered as freeware?

Public Comments

1. AutoCAD is one of the most popular, but can get really expensive, too. Try http://www.freecad.com/ or http://www.freebyte.com/cad/cad.htm for starters
2. autocad