The BME280 Atmospheric Sensor from Whadda is the easiest way to measure barometric pressure, humidity and temperature.

LPS331AP Pressure/Altitude Sensor Carrier with Voltage Regulator

Pressure Sensor MD-PS002 - Absolute Pressure

Life has its ups and downs, so why not measure them? The MPL3115A2 is a MEMS pressure sensor that provides Altitude data to within 30cm (with oversampling enabled).