Rich Geldreich, a losless data and texture compression specialist who formerly was working at Valve and involved with the Linux cabal while now is employed by Unity, wrote a blog post today entitled zlib in serious danger of becoming obsolete.
The reason for Geldreich's title is this: "in the lossless compression world we’re lucky to see a significant advancement every 5-10 years. Now, we have two independently implemented codecs that are giving zlib serious competition on multiple axes: throughput, ratio, and even code size." The two new lossless codecs he is talking about are Google's Brotli and RAD Game Tools' BitKnit.
The benchmark results he published show BitKnit and Brotli competing well with Zlib and he's also included results from LZ4 and his own LZHAM codec.
The Brotli compression format is already open-source via GitHub while RAD Game Tools will be releasing BitKnit with the next version of their software.