I know at least KDE's Baloo indexer uses tags on files when available. If I search for the word "cookie" I get every filed named cookie, anything tagged cookie,

I know at least KDE's Baloo indexer uses tags on files when available. If I search for the word "cookie" I get every filed named cookie, anything tagged cookie, contacts, bookmarks, and any parsable text document containing cookie including emails and IM logs. I think the broader problem is that there is this awkward question of where tags belong in the application heirarchy. Right now most media files embed tags while text files do not. Binary files certainly do not, and folders are not true files and thus are not on-disk things to put tags on, they are usually filesystem constructs in a data tree somewhere. Your file indexer could maintain the tags database, but then they are not file-portable. They could be part of the extended file traits in your filesystem, but then filesystems lacking support (including online uploads) would drop them. Thus we are at a point where tagging is done on a per-format basis, and thus some files are just not taggable but instead require indexing. You would probably want to throw a semantic relevancy engine on top of that to associate vocabulary with similar terms, so if i search for food i can get things like cookie as a related search term.