Sim Daltonism - From the perspective of a color blind person, some colors are impossible to distinguish.
LinkChecker - LinkChecker is a free, GPL licensed website validator.
Coblis - If you are not suffering from a color vision deficiency it is very hard to imagine how it looks...
DRKSpiderJava - DRKSpider is an open source website crawler (sitemap generator) and link checker. Features:
Colorblind Web Page Filter - Enter a URL, and this tool will render it as someone with color blindness will see it.
Pulno - SEO Audit and Website Analysis. Pull up your SEO.