pngview is a standalone PNG viewer that decodes PNG files without relying on libpng or zlib. pngview is written in C99 and uses Xlib to display the images on the user's screen.
Compile it e.g. like this:
$ cc -O3 -Wall -o pngview -std=c99 *.c -lm -lX11
or $ make
Then run it like this:
$ ./pngview testimage.png
pngview was written for the fun of the exercise, and is neither very efficient nor bug-free. It supports indexed pixels, grayscale, truecolor and alpha transparency, but probably not all permutations of the aforementioned. It assumes well-formed PNG files and a bit depth of 8 for each color channel.