I recently got a Dell 20" LCD to act as an external monitor for my MacBook through its Mini DVI-to-DVI connector. When working at night with only a small lamp for illumination, it was impossible to set the brightness low enough so that I wouldn't have to squint at the screen.
Inverting colors using Control-Option-Command-8 made it just slightly more comfortable, but I didn't fancy the weird colors. Calibrating the screen to a lower gamma didn't do much either. In the end, I found these two freeware utilities:
Mac OS X Hints