is a natty bit of software for testing what your maps (and other designs) look like to people who have various forms of colour blindness.
You can get it here.
In my opinion (or, for my degree of colour blindness), the simulation is very extreme, way too strong.
I personally prefer the Photoshop plugin here. I used it to explain my colour blindness to my wife. Apply the filter, then if neccesary Fade it (ctrl-shift-F) until the colourblind person cannot see a difference between the original and the filtered image. My wife was adamant that the before and after pictures (of a bunch of red roses) were very much different.