I use a program that allows me to share my mouse and keyboard with multiple machines (Basically for a multi-monitor setup, but each machine is completely independant). I was sad to find that the program I use, while absolutely phenomenal, doesnt allow the iMacs standard F1 and F2 keys to control each screen independantly, but rather only works for the machine the keyboard is tied to. However, this program does EXACTLY what I need; allows me to control each screen of my multi-monitor setup independantly from eachother, so I can have varying brightness across each system.
This application works so wonderfully, I would have paid up to $5 for it. Thats how much I love it, and how invaluable it is to my useage scenario.
To the folks complaining that the brightness is not constant and changes on its own; be sure to turn off Auto Brightness in your System Prefs > Display settings. The program works perfectly fine, they just have their Auto-Brightness turned on, which will effect this program.