The MPL3115A2 is a MEMS pressure sensor that provides Altitude data to within 30cm (with oversampling enabled). The sensor outputs are digitized by a high resolution 24-bit ADC and transmitted over I2C, meaning it's easy to interface with most controllers. Pressure output can be resolved with output in fractions of a Pascal, and Altitude can be resolved in fractions of a meter. The device also provides 12-bit temperature measurements in degrees Celsius.
This breakout board makes it easy to prototype using this tiny device by breaking out the necessary pins to a standard 0.1" spaced header. The board also has all of the passive components needed to get the device functioning, so you can simply connect it to something that talks I2C and get to work!