Vector graphics editor
We recommend Inkscape (download : http://www.inkscape.org ).

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Raster graphics editors
Raster graphics editors There is MyPaint for digital painting (download : http://mypaint.intilinux.com). The most of us use Gimp (download : http://www.gimp.org), althrough there are some others great freeware like Krita (download : http://krita.org).

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3D computer graphics software
How can we not recommend Blender (download : http://www.blender.org) ?

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We recommend too the open drawning project Alchemy, which is a sketching software, quite hard to define, however, it's a great tool to boost your creativity (download : http://al.chemy.org).