The Gravity Plug is a small board containing the BMA020 3-axis accelerometer sensor. The sensor has a high (10 bits) accuracy and a software-adjustable ±2/±4/±8g range (i.e. up to 64 LSB/g). It communicates directly over I2C.
There is a “GravityPlug” class in the JeeLabs Ports library to interface with this sensor and read out the 3 axes as integers. Make sure you get the latest version of the Ports Library as the Gravity class in the earlier version had a small bug in it that ignored negative numbers.
An I2C bus running at 3.3V is used as interconnect, with all the power and signal lines brought out to to both sides of the board to allow daisy-chaining with other I2C-conformant boards.
The Assembled version comes with the SMD-sized BMA020 chip and decoupling capacitor already soldered onto the board. Headers for this and all other JeeLabs plugs can be found here (straight) and here (right angle).
The PCB Only version contains just the printed circuit board for the Gravity Plug. No parts, no headers.