[curt.clifton]

Even better would be arbitrary meta-data with the option to expose each particular field in a column, or just in a meta-data inspector. For data that one might store with nearly every item, say tags, the column would be useful. But for data that might only apply to a small number of items it would be better not to spend the screen real estate on a new column. An example of this would be Mail message URLs. These would be manipulated by and used by scripts, so the user doesn't really need to see them.

Taking it a step further, a meta-data field like Mail message URL could optionally be mapped to an "indicator column". The indicator column would merely shows the presence of the related meta-data, much like the paper-clipped note icon does now.