For improving general-purpose gzip / zlib compression, there is the Zopfli project [1] [2]. It also has (alpha quality) code for PNG file format; since this functionality wasn't originally included, there are also third-party projects [3].
You might be able to shave a percent or so off the download size of compressed assets.
[1] https://news.ycombinator.com/item?id=5316595