Image stacking is also used in astrophotography. Consider it "time stacking", while what Jeff refers to is "focus stacking." Helene posted some great examples of the latter a good while back.
Not sure what software is used, but I believe it involved virtual strings and mirrors.
This shot looks like a panorama (photos stitched together), not a photo stack. I'm not sure what the benefit of stack would be on a wide angle landscape.