Adafruit MMA8451 Triple-Axis Accelerometer w/ 14-bit ADC. It has a wide usage range, from +-2g up to +-8g yet is easy to use with Arduino or another microcontroller. This sensor communicates over I2C so you can share it with a bunch of other sensors on the same two I2C pins. There's an address selection pin so you can have accelerometers share an I2C bus.
Dimensions: 21mm x 18mm x 2mm / 0.83" x 0.71" x .08"
Uses I2C repeated-start, address is selectable to either 0x1C or 0x1D