I didn't really think you could find something better than the Apple brightness tool, yet ScreenShade is significantly more powerful.
With this application, you can easily darken your screen completely or make it really bright.
The program is accessible from the menu bar and pressing 'toggle ScreenShade' adjusts brightness according to the level of light in the room.
Press 'Redetect external monitors' to reset it and get a new brightness level. I wasn't all convinced by the level of light that ScreenShade chose though as most of the time it simply was too dim to work properly.
Best is probably to open preferences and set brightness yourself, via the little slider. You'll see you can also select what areas to shade.
ScreenShade also works very well with two simple shortcuts: Option with plus or minus to adjust it yourself.
More powerful than the brightness controls in your Mac, ScreenShade is good to have around if you work in poor lighting conditions.