Zopfli Compression Algorithm is a compression library programmed in C to perform very good, but slow, deflate or zlib compression.
APACHE-2.0 License
miniz: Single C source file zlib-replacement library, originally from code.google.com/p/miniz
Collection of compressors, built upon a small and simple graph-based LZSS framework.
Gzip Decompression and Random Access for Modern Multi-Core Machines