There were a few anaglyph 3D images that I wanted to view as cross-eyed stereo pairs (I don't have glasses handy). I started doing the manipulations by hand to convert them from anaglyphs, then got tired of doing it manually.
So, I wrote a quick script-fu to do it for me. This will take a 3D Anaglyph image (red/cyan), and create a stereo pair (monochrome) for you...
Just load up an image, and run:
Filters > Generic > Anaglyph to Stereo...
- GIMP Version: