This project uses a HH10D humidity sensor (http://www.sparkfun.com/products/10239) and an Arduino. The HH10D is a little card with a sensor and a 7555 oscillator and outputs a frequency that depends on the humidity. It is calibrated when you buy it, the calibration values are stored in an onboard Eeprom. This is a I2C-connected Eeprom. The values has only to be read out once, then they can be hardcoded. Important: The HH10D uses 2.7-3.3V power supply!