Meet Luxi: a nifty device that turns your phone into a light meter. This is a perfect tool for folks who want to use their DSLR in manual mode. Just slip Luxi onto your phone to be able to determine what settings to use to get the perfect exposure. The device does not need a battery to work. It is not too bulky either.
Luxi is a time saver. Just bring your phone and DSLR, and you are set. Sets you back $30.