Macbook air screen is too bright even on the lowest setting Is there a tweak, hack, or piece of software that can adjust the brightness of the MacBook Air's screen below the settings that MacOS X allows by default?

In low light the lowest brightness setting is too bright for me (and the setting after that is black so that doesn't work at all). :-)
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.