file organization

It would be really helpful if I could make new folders and sub-folder, and also have them be virtual (so that a file can be assigned to more than one folder) If I start having many documents it's going to get difficult to manage all of them and organize them in meaningful ways - and a sidebar browser so that I can view all document types - I'd I'd really even like to be able to have a word processor document in one tab and a spreadsheet in another, without having to have different document types in different windows - .... usability critique from an ex-computer scientist turned designer, artist and researcher
earthspirit