Google "reinvented" the good old gzip... Read more at https://github.com/google/zopfli/blob/master/README.

Backwards compatible with gzip, but with better compression ratio. On the downside, it is very very slow during compression.

Zopfli

Get a copy from git and compile

git clone https://github.com/google/zopfli.git
cd zopfli
make zopfli zopflipng

Zopflipng

Optimize all PNG images down from the current directory.

for i in `find | grep .png$`; do zopflipng -my $i $i; done

-m maximal compression
-y overwrite the destination