Elyse seems to be the one I have been looking for. Love at first sight.
From their Website: "The concept of folders and directories is replaced in Elyse by tag nodes. Each node is somewhat like a saved search only much faster. Nodes are arranged in trees and give a similar effect to that of folders and directories. The difference is that the trees can be rearranged without any impact on how the files are stored or what tags are associated with each file. The same copy of one file can simultaneously appear in multiple overlapping groups of files, each group represented by a node. For example, you might have a photo of the family at the beach while on holidays. The same one file in Elyse can appear in a list of last summer's holiday photos and also in a list of photos of family members."
"Elyse also recognises that relationships exist between tags. For example: lions, ostriches and elephants are all animals. By creating a tag relationship structure to define these relationships it becomes possible to instantly create a list of all files that have an animal type tag associated with them. There is no need to attach the tag animal to every file about an animal."
It's available for both Windows and the OS X.
I will post more after I have used it extensively.