There are several applications and web services that help you determine if the colors used in your designs will be accessible to users that cannot view all colors. My personal favorite is the Sim Daltonism app for the Mac (pictured above).

1. Sim Daltonism is a color blindness simulator for Mac OS X. It filters in real-time the area around the mouse pointer and displays the result — as seen by a color blind person — in a floating palette.
2. Vischeck's color deficit simulator plugins for Photoshop allow you to simulate how color blind users see your screens.
3. etre's color blindness simulator is a web-based service that allows you to upload a screenshot and view the screen with color blind simulation.