In Ubuntu 11.10, you can run System Settings, Screen and then there should be a
slider to set the brightness. At least, there is on my laptop, which has Radeon
HD 4200 graphics and uses the AMD proprietary FGLRX graphics driver.
On my desktop, Ubuntu 10.10 with Nvidia graphics, I can run Nvidia X Server
Settings, and under the "X Server Color Correction" tab, there is a brightness
slider.