I like the GIMP lens flare, very good tools, I would also note I have a love of the LUCE photoshop plug in - works very well in GIMP with PMSP.

 

Edit: Adding the link -  http://amicoperry.altervista.org/luce/index.html

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.