BeOS file system with metadata for OS X
The next OS X release is rumoured to use the BeOS File System. This is the best bit (see also WinFS for Longhorn):
For the everyday user, though, BFS has much more tangible advantages. Any file or file type on a BFS volume can have arrays of metadata associated with it, in the form of "attributes." There is no limit to the amount, size, or type of attributes, and attributes can be displayed and edited, sifted, sorted, and queried for directly in the Tracker (Be's equivalent of the Finder). Because most attributes are indexed, search results are nearly instantaneous, regardless the size of the volume or the number of files being searched through. By default, BeOS ships with reasonable sets of attributes for common file types, but users are allowed to extend and customize these, and to create entirely new file types with entirely new arrays of attributes. In other words, the Be File System doubles as a database.