My exterior rear view mirrors have those "stick on" mirrors in the corner. They are fish-eye convex like the security mirrors in a store. Works well for a big truck mirror -they probably take up to much real estate on a small sports car mirror.
Gets low scores here:
Auto Safety Editors' Results: Best 2007 Car Gadgets & Automotive Safety Technologies
edit:
Took me awhile to firgure out what they were selling. Looks like they tap into power mirrors and let you scan the blind spot by "panning" the mirror with a seperate swith. Kinda cool but I wonder what the life cycle is of those motors and plastic gears?