I recently downloaded and am testing Photology
, a free download that indexes your photo collection and then lets you find photos by applying filters like faces, sky, color, location, orientation, and time of day.
It doesn't get everything right, but not bad.
The initial indexing takes a while because it not only indexes the file names and locations, but it must do some sort of analysis of the images so it can apply the filters. You tell it which folders to index, and then it watches those folders for new images and adds them to the index.
The program used to be offered for sale, but earlier this month it was made available for free (donations accepted).