Wikipedia
Zopfli
Zopfli is a data compression algorithm that encodes data into DEFLATE, gzip and zlib formats. Zopfli is regarded as the most size-efficient DEFLATE encoder available. In February 2013, a reference implementation of the Zopfli algorithm was released by Google as a free software programming library under the Apache License, Version 2.0. The name Zöpfli is the Swiss German diminutive of “ Zopf”, a unsweetened type of Hefezopf.