If you're writing new code, you probably want to check out Salza2, which is a from-scratch reimplementation, and is much faster and simpler (and maintained).
Salza is a Common Lisp compression library that provides an FFI-less interface to the ZLIB and DEFLATE compressed data formats.
Currently, Salza only compresses data, decompression is not implemented.
Salza's PNG functionality is one of the backends of Yannick Gingras's Fract
fractal explorer. CL-PDF also uses this library for compression of pdf files.
Salza is licensed under a BSD/MIT/X11-ish license.