JBtek BMP180 Barometric Pressure, Temperature and Altitude Sensor;

This precision sensor is the best low-cost sensing solution for measuring barometric pressure and temperature. Because pressure changes with altitude you can also use it as an altimeter.
The sensor is soldered onto a PCB with a 3.3V regulator, I2C level shifter and pull-up resistors on the I2C pins.
Using the sensor is easy. If you're using an Arduino, simply connect the VIN pin to the 5V voltage pin, GND to ground, SCL to I2C Clock (Analog 5) and SDA to I2C Data (Analog 4). Download the BMP180 Arduino library and example code for temperature, pressure and altitude calculation. Immediately you'll have precision temperature, pressure and altitude data.