The DHT-22 is a low cost humidity and temperature sensor with a single wire digital interface. The sensor is calibrated and doesn't require extra components so you can get right to measuring relative humidity and temperature.
This is a great digital sensor to use with any microcontroller or single-board computer. It uses a digital protocol (referred to as '1-wire') that doesn't require an analog input on the board taking the measurements.
Remember to connect a 4.7KΩ resistor between the data pin and power (as a pull-up) or you might not get any readings at all!