How colourblindness affects the viewing of web pages. Colour blindness affects approximately 8% of males and 1% of females, commonly by being unable to distinguish between red and green. Here is an interesting web tool for designers to check their sites.
Vischeck's colour vision model allows you to simulate how the world looks to people with various sorts of colour deficiency. You can check a single image, or webpage, or just download it. Here's a temporary snapshot of MeFi after being vischecked.