BeOS had a nice idea, although it still used files. It's file system doubled as a database, and you could add arbitrary columns to files. For example, contacts were empty files in a special directory, and had attributes like "address", "name" and so on. Mails were also stored in plain files, but unlike mbox/Maildir all the metadata was stored as attributes, not in the files, making it easy to process them with scripts, or to sort them in the file browser.

Its a lot like the never-finished WinFS from Microsoft.

Funnily, modern file systems (extfs3/4, NFTS, HPFS+?) all support extended attributes in some form or another. However, they are currently only rarely used: Mostly for the "this file was downloaded from the internet, do you really want to open it" flag. I wish more programs would use them to store interesting metadata, but it's basically a chicken and egg thing now.

Windows and GNOME also have concepts where you can have calculatable attributes - you have a little library that looks up metadata in a database or parses it from the file, and then serves it as an additional attribute on the file (visible in the file properties tab). You can see it e.g. on mp3s or word documents in windows. However, it doesn't seem to be widely used either, and I wouldn't be surprised if that function has been gutted out of GNOME lately.