pngmerge -- composing APNG from PNG images.
After 3/21/2009, I've written a python script to compose APNG from PNG images. This is just a stand-alone script using only native python functions. The code is on http://kichiki.web.fc2.com/pngmerge/.
Here's an example (http://www.ualberta.ca/~ichiki/images/stokes-flocs2-6e.png):