File system that uses tags rather than folders?

Here are some file systems which I found using google.

TagFS - "Tag Semantics for Hierarchical File Systems" paper by Stephan Bloehdorn and Max Völkel, 2006 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.60.4187

dhtfs - "Tagging based filesystem, providing dynamic directory hierarchies based on tags associated with files" a usable implementation, last release 2007 http://code.google.com/p/dhtfs/

Tagsistant - "A reasoning semantic filesystem for Linux and BSD" project under active development http://www.tagsistant.net/

Leaftag - "Tagging for the Linux desktop" another implementation, last release 2006 http://www.chipx86.com/w/index.php/Leaftag

On integration with OSes, I dont think it should be that difficult. OSes are deeply dependent on file system hierarchy's but tag based file system can mimic directory structure. For example in a tag based file system the path /etc/init.d will give all the files that are tagged with exactly two tags i.e 'etc' and 'init.d'. For files which are tagged with other tags as well as these two tags, their extra tags can appear as directories inside /etc/init.d. If there is a file a.txt with three tags i.e 'etc', 'init.d' & 'asdf' then 'asdf' will appear as directory inside /etc/init.d and full path of a.txt will become /etc/init.d/asdf/a.txt.

I'm the author of the Linux program TMSU that lets you tag files and view them in a tag-based filesystem.

Here is some example usage:

$ tmsu tag track.mp3 music genre=indie-rock good year=1999
$ tmsu tag melody.mp3 music
$ tmsu tag DC123.jpg photo landscape country=italy year=2014
$ tmsu files music year = 1999
./song.mp3

And the virtual filesystem:

$ mkdir mp
$ tmsu mount mp
$ ls mp
query  tags
$ ls tags
country  genre  good  landscape  photo  music  year
$ ls tags/music
genre  good  track.1.mp3  melody.2.mp3 year

The numbers in the filenames are identifiers which are there to keep the filenames unique. These are symbolic links back to the files' original locations.

$ ls tags/music/good  # you can drill down
genre  track.1.mp3  year
$ ls "query/music and good"  # this directory gets automatically created
track.1.mp3

It's still in active development. At time of writing (Oct 2014) 0.4.2 is available but I'll be releasing 0.5.0 soon.