Adafruit MMA8451 Triple-Axis Accelerometer @ 14-bit ADC with wide usage range of ±2/4/8g and easy to use with Arduino or another microcontroller. Detect motion, tilt, and basic orientation. A great accelerometer to start with. 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. Please note this chip requires repeated-start I2C support (in case you are looking to port this to another processor)

The MMA8451 is a miniature little accelerometer from Freescale. Designed for use in phones, tablets, smartwatches, and more, but works just as well in your Arduino project. MMA8451 is the most precise with a built-in 14-bit ADC. The accelerometer also has built-in tilt/orientation detection so it can tell you whether your project is being held in landscape or portrait mode and whether it is tilted forward or back