Anyone who knows anything about studio photography knows that taking an incident light reading is the best way of getting accurate exposure metering, without having to adjust for the reflective qualities of your subject (that means you, snowmen and black cats). This is a really-neat accessory that clips right over your iPhone's front-facing camera and turns it into an incident light meter. $30, plus an extra $9 to ship to the UK (I make that about £23 at time of writing).
Serious photographers sometimes spend hundreds of dollars on an incident light meter–a small handheld device that measures the light falling on the subject. Luxi is a $30 iPhone accessory that promises to do the same job using just your existing phone.