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. ASDF-install package (obsolete)

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.