GitHub - google/zopfli: Zopfli Compression Algorithm is a compression library programmed in C to perform very good, but slow, deflate or zlib compression.

https://github.com/google/zopfli

Zopfli Compression Algorithm is a compression library programmed in C to perform very good, but slow, deflate or zlib compression.

Google Open Source Blog: Introducing Brotli: a new compression algorithm for the internet

http://google-opensource.blogspot.jp/2015/09/introducing-brotli-new-compression.html

While Zopfli is Deflate-compatible, Brotli is a whole new data format. This new format allows us to get 20–26% higher compression ratios over Zopfli.