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
https://github.com/mayuresh/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 don’t 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.