Have you ever tried to dim your iMac display? Apples iMac default brightness controll is not very satisfying.

Charcoal Design Shades is a little application that gives you fine control over how bright or dim your iMac's display is. It runs in the background providing always-available fine-grained control over the brightness of your display via a Mac-like interface. It performs this without actually changing the real brightness of your display. It applies an opacity filter to your Desktop similar to Splasms Software Brightness Control

Both tools have no power saving effect! Shades is available as ppc, intel and universal binary.