Precisely Adjust Display Brightness
Hold down the Option+Shift keys while you adjust screen brightness to precisely adjust the displays brightness by allowing you to have 64 levels of varying brightness vs the default 16.
The incremental adjustment allows for precision when setting a screen brightness level, offering tremendous control over exactly how bright or dim you want the Mac display to be.
As you can see in the screenshot, this is done by breaking each brightness step into four stages. I find this kind of precision really useful on my MacBook Pro when in dimmer lighting conditions, but it works on desktop Macs too, and with just about every version of OS X.
You can do the same type of precise adjustments when changing your Mac volume levels too, which also offers the incremental controls through the same keyboard modifier trick.
Note that new versions of OS X still allow for this feature, but with Lion through Yosemite a slightly different modifier key is required for the incremental precision adjustments using the function keys for volume levels and brightness levels on the newest Mac keyboard.