The Wii MotionPlus (Wiiモーションプラス) is an expansion device for the Wii Remote video game controller for the Wii that allows it to more accurately capture complex motion. According to Nintendo, the sensor in the device supplements the accelerometer and Sensor Bar capabilities of the Wii Remote to enable actions to be rendered identically on the screen in real time. It was released in June 2009.
In November 2009, a black Wii Motionplus was released to coincide with the release of the black Wii console.
Source: Wikipedia, "Wii MotionPlus", available under the CC-BY-SA License.