HLLib is a package library for Half-Life that abstracts several package
formats and provides a simple interface for all of them.  HLLib is written
in native C++ but exposes both a C and C++ interfaces which can be used in
any C or C++ application (additional languages may also use the library with
standard C imports).  HLLib works natively in both Windows and *nix.  BSP,
GCF, NCF, PAK, SGA, VPK, WAD, XZP, and ZIP (store/deflate) package formats
are supported.

WWW: http://nemesis.thewavelength.net/index.php?p=35