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?
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.
There is TagsForAll for windows. It is a file manager based on tags. Tags can have hierarchical structure. User interface is very simple but nice. Free version fully functional and save tags in database, Pro version save tags also within NTFS stream to a file.