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), although there are some other great freeware versions like: Krita(download: http://krita.org).

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

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We also recommend using the “open drawing project Alchemy”, which is a sketching software, it's quite hard to define however… but it's a great tool to boost your creativity(download: http://al.chemy.org).