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.
Most internet users have monitors that can display more colors than the 216 that are used in the traditional “browser-safe” palette. moreCrayons is a bigger box of crayons; 4,096 colors for the web. A site by our own kirkaracha. [Via Zeldman]