Mechanical gear shapes can be created easily in Inkscape. This tutorial will show you how, and give you an idea on applying filters to make the finished design look old and worn.
Link to Inkscape Filter Guide: http://www.littlewebhut.com/inkscape/graphics_filter_guide/
Fast Forward past the first 45 seconds of this video.
Learn how to use the iwarp filter for squishing, stretching or growing images in weird ways. Good for making funny faces.