I just found this plugin for Gimp:
http://registry.gimp.org/node/15994. It applies a comic effect. To use it, click on the comicbook.scm link, copy the text and paste it into Notepad. Go to save as in the file menu. Where it says save type as, select all files and put .scm after the name of the file. Save it in the folder c:\program files\gimp 2.0\share\gimp\2.0\scripts folder. The effect looks like this:
