Smart Folders (Finder -> File -> New Smart Folder). It's not exactly hidden but I never paid too much attention to it. It's essentially a way to create a folder whose content is dynamically updated based on your search conditions. For example, you can create a smart folder that contains all PDF files matching a certain name pattern within a given directory (or the whole disk) [0]. You can get really advanced, there's…
I would LOVE them if they were reimplemented as a fuse style file system instead of a userland macOS trick.