I've wanted to apply filters retroactively in Gmail (i.e., if I've got 10MB of mail in GMail and now I want to organize it into labels, I create the labels, create the filters to auto-archive and set labels, but how do I apply the same filter to old mail?).
There didn't seem to be an obvious way to do that. But then that's because I was thinking about it as "applying filters". So naturally I was looking for it in the Filter stuff (create, edit, delete, test, etc).
As it happens, there'ss a simple way to do retroactive filters in GMail. Just search for the relevant email using the search function. It'ss not going to be exactly the same as filtering, UNLESS one uses the "Show search options" link. That brings up a dialog box similar to create a filter, except it works only for one search.
After the relevant emails have been found, it's a simple matter to select them all and apply a label and (i like to do this so they go away from the inbox but are still in the label/folders) archive them. It's not quite as easy as it might be, but it works. Now I just need to remember that that's how it's done.