#2 Kuwahara/Symmetric Nearest Neighbour

[Marcin Chara]

http://pippin.gimp.org/image_processing/chap_area.html#id2534703
describes few simple effects, Kuwahara & Symmetric Nearest Neighbour are quite simple and gives quite effective results so you can start with those two.

You can also take a look at other chapters at http://pippin.gimp.org/image_processing/
It's quite good page so perhaps you'll find inspiration and pick next effects to be implemented from there.