The DHT22 (AM2302) Temperature and Humidity Sensor
The DHT22 is a low-cost ($2) digital temperature and humidity sensor. It uses a capacitive humidity sensor and a thermistor to measure the surrounding air, and transmits a digital signal on the data pin.
These devices are easy to use since they have an internal analog to digital conversion. They are good for 20-80% humidity readings with 5% accuracy and 0-50°C temperature readings with ±2°C accuracy. They are designed to be polled about once every two seconds.