DHT11

The DHT11 is a digital humidity and temperature sensor. It measures in integer values, there's no kernel module (for the Pi), and it's about 5€ a piece. Good for 20-80% humidity readings with 5% accuracy and for 0-50°C (rather unpractically for weather-related applications, no temps below zero...) temperature readings ±2°C accuracy.

Learn more:

• RaspberryPi Humidity and Temperature Sensor
• http://www.adafruit.com/products/386.