Windows/Mac: Because files can fit in many different categories and contexts, a tagging approach to managing them - like the kind we have for photos, bookmarks and blog posts - makes sense. Free file management app Elyse provides tag management for your files.Although Windows 7 has tagging for photos and Office documents, and OS X has support for metadata (tags) that you can access with Quicksilver or create in the file info screen, tagging utilities and alternative file managers usually work best for more robust and universal tagging.
Elyse works on both Windows and Mac and has several file-tagging features. Besides creating new tags in the Elyse database for applying to your files, you can group tags together for easier management. To apply a tag to your files/folders, drag and drop them from Windows Explorer or Finder onto the tag.
One of the most interesting features of Elyse is autotagging: drag a folder from the program's file browser dock to a tag and your folder names will be applied as tags automatically to all files within the folder.
The program definitely takes some getting used to, but is an alternative way of categorising your stuff - regardless of its folder location. You can add, extract, delete and arrange files, which are stored in the database, but can't modify them directly from within it. If all you want to do is find a specific file or types of files, Windows and OS X's built-in search are easy and powerful enough to use. But for more flexible and contextual grouping of files, an app like Elyse could be very helpful.