This is a generic Wii Nunchuck controller, we haven't tried it with a Wii but it does work great with the Video Game shield, and all the microcontroller code we tried.
We suggest getting a Nunchucky breakout board if you want to use this with an electronics project.
There's a 3-axis accelerometer inside as well as a resistive 2-axis joystick and two buttons. You can grab the data over two i2c data lines. There's tons of example code for all sorts of microcontrollers for these guys!
See our Distributors page for a complete list of distributors.