"This is a 3-axis digital compass board based on the Honeywell's HMC5843. Communication with the HMC5843 is done through an I2C interface. The board has an I2C translator and 3.3V power regulator that will easily let you use it with 3.3V and 5V applications using a solder jumper. This magnetometer can be used on our ArduIMU+ and ArduPilotMega Shield or any microcontroller." ($44.90)
"The D11 version of the SCP1000 is similar to the D01, but it communicates via a two-wire (I2C) interface, rather than SPI. The SCP1000 is the first absolute pressure sensor on the market to use MEMS technology.
This sensor has a 30kPa-120kPa measurement range at up to 17-bit resolution. Under ideal conditions, this sensor can detect the pressure difference within a 9cm column of air. On top of absolute pressure measurements, the SCP1000 can also measure and report temperature in the range of -20 to 70°C. The pressure and temperature output data is calibrated and compensated internally, and the communication between the SCP1000 and its host micro-controller is realized using an I2C interface." ($34.95)
"This great tiny board will allow to interface slave 3.3V I2C devices like magnetometers and pressure sensors with master 5V devices like AVR/PIC microcontrollers WITHOUT THE USE OF DIRECTIONAL PIN. It also features a build-in 3.3V power regulator and dual pull-resistors (5V and 3.3V sides). The pull-up resistors can be optionally and independently disabled with the two solder jumpers located on the top of the board.
But why do you need it? Well some 3.3V sensors are not capable to handle 5V signals coming from some microcontrollers (like Arduino), you always can do it but you will reduce the life span of your sensor. Not just that, you will also need to supply the I2C sensors with 3.3Volts, so an external power regulator is required and just that makes things more complicated. All this problems are solved with this little board. " ($12.90)
And finally, one just for fun:
"Limited Edition Collectible Ardupilot MEGA and Shield (V1, V2) Prototype BoardsYou get 3 boards: MEGA, V1, V2. They are fully functional if you want to build your own. OR you can use them as paper weights, coasters, magnets, souvenirs or any other creative application." ($1.95)