TT’s Zip Package is a set of classes for managing Zip archive files, for OS X, Windows and Linux, without the need of plugins (on Windows, it requires a popular and public DLL file, though).
It provides two different APIs that allow both access to the lowest level of zip archives as well as a higher level that functions like FolderItem objects inside a zip archive.
Version 3.3 has been updated to work with Xojo (i.e. the demo doesn’t throw any exception due to access of the UI from a background thread) and fixes a few smaller issues.
Detailed version history: http://files.tempel.org/RB/About%20Zip%20Package.html
Web Site: http://www.tempel.org/RB/ZipPackage
TT’s Zip Package can be tried for free (inside the IDE) but requires a purchase (US$35) for use in built applications.