Thermal imaging cameras are capable of sensing heat like a traditional camera senses light. They're very cool, but extremely expensive, so modder Andy Rawson developed a way of attaching thermal sensing software to his iPhone. For about $150 in hardware, he outfitted his iPhone with a sensor capable of showing a 64 square grid of temperatures, feeding information through the dock port. While not quite up to the resolution of a professional device, the video below shows it off in action, and it does an admirable job of capturing areas of hot and cold.
Rawson has said he'll release the plans for the gear as an Open Source Hardware project so you can put it together yourself, and the iPhone software is ready with an Android version in the works. While it might not be able to tell you the exact temperature fluctuations in your oven, it will definitely expose the drafty spot in your house.
So, how long before this pops up on Kickstarter?