On Mac OSX you can right-click a file in the Finder and chose 'Get Info'. That brings up a dialog box that shows you the file attributes and lets you change selected attributes. It also has a section for 'comments', which sort of allows annotation.
But it is crap to use. I can add a comments column to the Finder window, but it isn't editable there. In order to add or edit a comment it's a minimum of three clicks.
Separately OSX Finder allows me to associate 'tags' with files fairly easily, but these tags are actually just colored dots, not arbitrary text values, and I can't search on them.
The underlying functionality is there and, on OSX, it is even used in a rudimentary way. But not in a useful way, so no one uses it.
What I want is a file browser that supports plugins for editing, displaying, and searching arbitrary data associated with files, with easy to access menu/toolbar support.
I take back one thing I said earlier: you can have arbitrary tags in OSX 10.9 or higher. But the way the UI works makes managing more than a dozen tags a real headache. And I still can't easily do text annotations, nor can I do links or picture annotations.
However, tagging in later OSX is possible and I think the search supports tags.
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