About 8-9% of all men and 1% of all women are “color-blind” (reference). If you ignore this while designing an application or website you're ignoring nearly 4 million people in germany alone.
But how can one check whether the application/site is useable by everyone?
Luckily Bernhard Jenny from ETH Zurich has developed a little tool for Mac OS X (windows and linux are supported via a Java-Application) to simulate the effect of three types of “color-blindness” (Deuteranopia, Protanopia, Tritanopia).
Color Oracle is a very simple menu-bar app. Just select the type of color-blindness you want to simulate and the current screen is filtered accordingly. It's that easy.
Check it out.
Update: I just read that Sim Daltonism offers similar functionality in real time but only for a small portion of the screen. Haven't tried it out, though.