I write this article as a work collegue asked me if there are generic utilities to adjust monitor brightness, so i look for them and here there are the best (free) i found:
Desktop Lighter for Win XP, Server 2003, Vista; you can activate features with hotkeys
Display tuner for Win 2000, XP, Server 2003, Vista; adjust settings (not only brightness) easily with sliders
Shades specific for Mac (PPC, Inter, universal)