Mac OS X: While the function keys on a Mac will always be the easiest way to quickly dim or brighten the screen, Brightness Slider gives you precision control from your taskbar.
Brightness Slider doesn't do much else, but what it does do, it does well. The slider gives you more accurate control than the function keys do and you can drop it a bit lower than Apple's preset, which comes in handy in those moments when you're working in pitch black. It's a free download in the Mac App Store, and if you've ever wanted better control over your Mac's brightness, it does its job well.
Brightness Slider [Mac App Store]