[Semiapies]

Besides others mentioned, Sim Daltonism is a good OS X app for testing how an app or page's color choices will look with various forms of color blindness.

Color Oracle is a cross-platform app for the same purpose, though I haven't used it much, yet: http://colororacle.org It seems rather good in my limited testing.

Colour Contrast Analyzer is a Windows/Mac app for testing whether text/background contrast is sufficient. This is a matter of both color and brightness - a color combination that looks fine for a normally-sighted user may he completely unreadable for others for a variety of reason. (Part of why everyone's suggesting designers avoid the gray-on-gray or light-gray-on-white design hipster crap.)