Smart Dust is a new technology. Researchers are developing a network that consists of very small sensors that would float in the air like specs of dust. (Currently they are 5mm on a side and cost $5. They are working on getting them to 1 mm and $1.) These “motes” or smart dust would be able to notice things about their environment and communicate it back to a central location. They might sense things like traffic problems, weather (temperature and humidty), the movement of people, etc.
Michael Crichton’s science fiction book Prey is built on the idea of smart dust that gains sentience.
You can read more about Smart Dust here: