Maybe I'm confused, but I think the "folders" in Google Docs behave the way you'd like. I can put a doc in one or more folders, and then I can look at the conte

Maybe I'm confused, but I think the "folders" in Google Docs behave the way you'd like. I can put a doc in one or more folders, and then I can look at the contents of only one folder. And I can have sub-folders. And really the folders are just tags.