Macbook air screen is too bright even on the lowest setting
I use and recommend Shades (free), which can darken the screen beyond the built-in brightness control.  If you really want to get it darker, use the built-in shortcut Cmd+Opt+Ctrl+8 to invert the screen colors (same command reverses the effect).

Nocturne (free) allows you to tweak some of the inversion settings.