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:
Bend a image or panorama into a circe using the polar coordinates filter.
Create a grey planet and give it a red exploding effect.
Give your text an icy look using- cloud render, blurring, and blue coloring.
Combine a white to black gradient with a wind filter to form a brushed metal look.
The best tutorial on making realistic looking tattoos on skin (or any surface). This works with black/white and color tattoo's.
Keep foreground objects in focused and clear while making the background blurry.
Erase tattoos, jewelry, and objects from photos using the clone tool.
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.
Demonstration on how to apply filters to shapes.