1. Luxi ($24.95)
A good example of this is Luxi, an add-on that lets you use your iPhone as a light meter, a device that typically costs several hundred dollars.
Extrasensory Devices launched the product on Kickstarter, raising more than $120,000. Here you can find a clear explanation and demo video of what Luxi can do. According to the explanation, your iPhone's built-in light meter measures reflected light and uses a setting appropriate to the average of the whole scene. But it's often not the optimal setting, such as a sunset portrait, in which the foreground object or person is typically underexposed. A professional light meter, called an incident light meter, captures all the light falling on a subject and works much better in situations such as a sunset.
Luxi should be out soon. You can pre-order it for $24.95 on the Extrasensory Devices website. This is just one of a number of sensors the company is developing. Its focus is niche iPhone products for professionals. Another one they offer is Backbeater, a sensor that connects to a drum so drummers can track their tempo precisely and see if they're drifting from the tempo.
2. Lapka ($220)
Another enterprise focusing on developing sensors is Lapka, a Russian company obsessed with expanding the functionality of the iPhone. It has developed a $220 set of sensors that detect radiation, electromagnetic fields, humidity, temperature, and even how organic your food is. You can read more about it on The Verge. The most practical may be the sensor that measures electromagnetic fields. Many people are eager to locate their bed or their baby's crib in an area of their home where the radiation from all of their electronic devices is the lowest.
The other radiation meter is a geiger counter, which isn't all that practical, but Lapka claims it's as accurate as a military geiger counter, though it does take longer to get a reading. The professional-level humidity meter is useful for situations in which equipment is sensitive to humidity levels. The device also lets you quantify what humidity levels are best for your sleep or work, and then reproduce them. It's also useful for mothers of newborns who are often eager to create an optimal environment. The organic food sensor doesn't really tell you whether food is organic, but it does indirectly measure nitrate content, which is a good indicator of whether the food has been exposed to nitrogen-based fertilizers.
3. Node ($149)
Node is a device that connects via Bluetooth 4.0 to your iPhone and, in the words of Phonearena, brings a "bucket load" of sensors to your iPhone. It's still in the early stages, having recently raised nearly $75,000 via its Kickstarter project. The company describes Node as a "wireless sensor platform." It has plans for many modules, the first expected to be Luma, an 8-LED light package that you can use as a flashlight or with your iPhone's camera as a flash, and Clima, which measures barometric pressure, wind speed, temperature, and humidity.
I think we're going to be seeing a lot more of these sorts of add-ons in the future.