I was recently contacted about availability problems with the code for pngtools. Frankly, I’m mildly surprised anyone still uses this code, but I am happy for them to do so. I have resurrected the code, placed it on github, and included the note below on all relevant posts:
A historical note from November 2020: this code is quite old, but still actively used. I have therefore converted the old subversion repository to git and it is hosted at https://github.com/mikalstill/pngtools. I will monitor there for issues and patches and try my best to remember what I was thinking 20 years ago…