Nested tags for me, with tag tree viewable as a 'folder' structure in a side panel. Solves the problems "Which [one & only one] folder do I put this in?", "Which folder _did_ I put that in?!", and "Why are we allowing x copies of this in n folders, each with different dates/contents?".

Works for me with email, too.

Mutually-exclusive folders for-the-lose. Wellll…for my usage, anyway.