Operating System with tag-based filesystem

Is there an OS that, instead of a hierarchical filesystem based on directories, uses tags? A problem with directory hierarchies is that a file often belongs into more than one category. Some systems solve this with symbolic links and whatnot where a file appears to exist in two places at once. But is there a system where this is the default behavior? Where all files exist in the same place, but are differentiated and navigated by their tags?

There have been a few attempts - beos came to mind, as did winfs.

If someone was to implement a metadata filesystem it would probably use something like filesystem resource forks (on OS X) and alternate data streams on windows

You need not to migrate from your original operating system to satisfy your needs. Give TMSU a try. It is a FUSE mountable file system level solution to your problem, said to be cross platform compatible. I myself have not tested it yet.

As long as the file system hierarchy is in a single file system, you can use hard-links and your files will belong to each "tag" directory.