Sim Daltonism - From the perspective of a color blind person, some colors are impossible to distinguish.
BrokenLinkCheck - Free Broken Link Checker / Online Dead Link Checking Tool - find bad hyperlinks
Coblis - If you are not suffering from a color vision deficiency it is very hard to imagine how it looks...
Dead Link Checker - Check for broken links on any website
Colorblind Web Page Filter - Enter a URL, and this tool will render it as someone with color blindness will see it.
LinkChecker - LinkChecker is a free, GPL licensed website validator.