Thermodo, is a product designed by Danish developer Robocat. The companies first foray into hardware comes in the form of a tiny passive temperature sensor that plugs into your headphone jack of your smartphone or tablet.
The startup is primarily known for their weather apps and often would receive 1 star reviews on the app store with users complaining that it was not measuring the current temperature in the room ""I checked the temperature in our bathroom and then went to our fireplace and it still said -2 degrees!!1111 It doesn't work, it's a waste of time. 1 star!!". These reviews eventually led the team to develop the product idea.
Thermodo works by sending an audio signal through the temperature sensor. More specifically "This sensor will then attenuate the signal amplitude depending on the actual temperature. This attenuation can now be detected on the microphone input and through software we calculate the corresponding temperature."
The power drain to your phone is less than a set of standard headphones, and when not needed the sensor comes with a matching keychain carrier where it can be stored.
More details about the product can be found at: Thermodo.com/ or by following the team at @thermodo
Additional: Kickstarter page, HackThings, Mashable, Techcrunch, Gizmag, Engadget