Turn your iPhone into a pro-quality light meter with Luxi
If you ever struggle to choose the right exposure settings on your camera, but you don't want to pay for (and lug around) a separate light meter, you might want to check out a Kickstarter project called Luxi.
Luxi is small diffusion dome that clips over the front-facing camera of an iPhone to serve as a light meter.
When Luxi is released, you'll be able to use it in conjunction with a free light meter app to choose the best exposure settings on your DSLR or other camera.
As explained on the Kickstarter page, Luxi can help you improve the quality of your photos because it measures incident light, not reflected light, and thus won't get tricked into underexposing a strongly backlit subject.
Luxi is patent pending and will retail for $24.95.
Here's a brief video introducing Luxi.