Postwork & Filter Forge reminds me of  http://www.darksim.com/ DarkTree is a lot of fun.

zbrush has a lot of killer shaders effects.Don't know how feasible it would be to use Zbrush thow.

http://www.topazlabs.com/ like a lot ,fast ,easy helps a lot.

Don't know if it works with gimp ,but it works with photoshop elements & http://www.irfanview.com/ :)

I am a fan of SeanE and the hand drawn comics styles 
http://www.renderosity.com/mod/gallery/browse.php?user_id=23561

For a solo artist to hand draw comics like Diamond 005 ,006 takes to long ,a week a page is just not feasible but with renderosity and some killer fillters we might just get some kool comics :)

 

 

 

 

 

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The Artist that will fight for decades to conquer their media.
Even if you never know their name ,your know their Art.
Dark Sphere Mage Vengeance