Roughly 10% of men are color blind to some degree. You need to be sure your interfaces are accessible to them. (Unless you are designing exclusively for women I suppose, since women are about 20x less likely to be color blind.)
Sim Daltonism is the best way to test an interface on Mac OS X I’ve seen.
Here is a web-based colorblindness simulator. Here is another. Personally I prefer a native program though. It’s faster and more versatile.
If you are curious, you can test yourself for colorblindness. I have no idea how accurate that test is, but since different displays and operating systems usually show colors differently I’d be a little skeptical.
ADDITION 2009-10-11: WeAreColorBlind.com is a website dedicated to design patterns for the colorblind.