Program Whose Time Has Come: virtual folders, collections, file baskets...

What would be the one program you would like to have most that is already possible but doesn't quite seem to exist yet? A Program Whose Time Has Come? For me, it would be a powerful sidekick to file managers: a program to manage virtual folders, a.k.a file collections, a.k.a file baskets/organizers... Hope someone comes up with a catchier name!

The time when the simple folder hierarchy ceased to suffice is long gone, isn't it? I've seen plenty of requests for such functionality on various forums, but strangely no real takers yet. I'll describe my experience with the existing solutions, pretty immature all, but first, here's what the program would do:

A virtual folder is a folder that doesn't exist physically on disk. It is merely a name for a collection of files selected by the user for whatever purpose or gathered automatically according to some criteria. The "whatever" is pretty darn big - I come up with new potential uses for such a program every day. Here are the three main uses of file collections I can see. (Please add others if my three don't exhaust the range of possibilities). Each of these uses requires slightly different behaviors, but it seems possible to accommodate them all in a single application.