I know some here are using Macs. Just came across this software today by accident. It's fantastically simple, and best of all free. Saves having to put red gels over your screen.
Seems to convert the display to grayscale, then cuts green and blue channels so only the red is truly displayed. You can set it to darken or lighten, plus it seems to have other features I haven't played with yet.
RedScreen for OS X