Tools for Seeing Valueshere at Dick Blick.
Accuview App - Great app that is primarily an app for composition but it has the ability to be used as a value finder. Just take your photo and use the value scale to remove color. Easy to use. For iPhone, click here.
Value Sketches - A tried & true method. It's not an app and maybe not "sexy" but I think it's still the best way to be a good artist. Learning to see as an artist requires training your eyes. Using tools and apps can be good but there's no substitute for seeing it for yourself. So grab your sketchbook, look, look again, judge one value against another & soon it will become a valuable skill that will carry you along your journey of becoming a better artist.