David Revoy is an extremely talented illustrator and a long time supporter of free & open source tools. He released "Chaos & Evolutions"[1] a few years ago where he shows his workflows in gimp, alchemy and mypaint, he was involved with the making of the movie "Sintel", another open source project made in Blender[2] and his blog is filled with tutorials for the Krita software (photoshop alternative for ilustrations). You can find a lot of his wonderful works on deviantart, but my favorite by far (and I urge everyone to take a look) is the graphic novel "L'heritage en couleur" [3].
[1] http://www.davidrevoy.com/article44/artworks-from-my-dvd-cha...
[2] https://durian.blender.org/
[3] http://deevad.deviantart.com/art/l-heritage-en-couleur-30812...