C.S. Muncy does a wonderful review over at The Photo Brigade of the same app I use on my iPhone: Pocket Light Meter. When I'm not using my D7000 in shutter priority mode, I need a quick and efficient way to gauge the amount of light available to me for a given shot. I decided a while ago that incurring the expense of a light meter was just not practical, nor was its use out in the field (being as that none of my work is produced in a studio where I can control the amount of light on my subject). The Pocket Light Meter app fits the bill nicely and if you're serious at all about capturing the perfect shot, I suggest you give it a try.
The Photo Brigade: Pocket Light Meter App Review by C.S. Muncy