Blender is an Open Source tool for creating 3D Models, Games, Movies, Stills, and Animations... Blender does have a somewhat steep learning curve and can be a bit difficult at first.
Some of my own examples are:
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Inkscape is an Open Source Vector program. It is rather similar to programs like Xara and Adobe Illustrator. Rather simple to learn, but can be a bit tricky at first... like most things.
Some of my examples are:
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Hope this helps a bit.