BrightnessSync is a preference pane for Mac OS X.

When you own a new MacBook Pro with a Mini DisplayPort and a LED Cinema Display, you have noticed that you can control the brightness of the internal display of the MacBook Pro with the brightness keys of the keyboard. And in the Displays preferences, you can enable automatic brightness adjustment for the internal display. To modify the brightness of the LED Cinema Display, you always have to use the Displays preferences, and the brightness of the external display can't be adjusted automatically. BrightnessSycn eliminates this shortcoming: it enables automatic brightness adjustment for both the internal and the external display.

And because the LED Cinema Display is more brilliant than the internal display of the MacBook Pro, BrightnessSync lets you set brightness levels for different ambient light conditions for the two displays independently. With this, there is nearly no need for manually adjusting the brightness for one of the both displays.

Requirements:
OS X 10.8+
Intel

Also available is:
Universal Binary version 1.0.1 for OS X 10.5-10.7