Google "reinvented" the good old gzip... Read more at
https://github.com/google/zopfli.
Backwards compatible with gzip/gunzip. Zopfli provides a better compression and with a similar or slightly better decompression speed. On the downside, compression is very very slow.
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