I'm currently doing a little home project that involves reverse engineering the .unitypackage file format. I have a lot of free Asset Store assets downloaded, and find the built-in categorisation system useless for finding what I need. Therefore I'm making a .NET tool that runs through my asset store folder and stores package names, paths, filenames and preview images in a SQL database so I can do full-strength searching. This would allow me to (for example) find all *.anim files, or all model formats that contain "car" in the path.
It likely wouldn't cover all facets of the file format, just the ones I need for my project. Anybody interested in an article when I've finished my little project?