I2C Pressure Sensor
The BMP085 (Datasheet) from Bosch Sensortec is a simple, easy to use, pressure sensing device. It speaks I2C and includes a temperature sensor. Because of it's tiny size of 5.0mm x 5.0mm x 1.2mm it fits nice into the Freerunner.
The sensor needs just four wires: power (3.3V, GND) and I2C (SDA, SCL). The wiki page about I2C describes where to get them.
I glued mine next to the BT antenna.
An experimental kernel driver is available from gitorious. It searchs for the chip at I2C address 0x77 and exports a sysfs interface where you can read the temperature and the pressure as well as adjust the internal oversampling.
--Cmair 19:10, 30 September 2009 (UTC)