tota11y - An accessibility visualization toolkit from Khan Academy
accessiBe - Making websites accessible to people with disabilities
Nibbler - Free tool for testing how good your website is, and what you can do to improve it. Check accessibility, SEO, social media, compliance and more.
Sim Daltonism - From the perspective of a color blind person, some colors are impossible to distinguish.
Page Speaker - Modern web accessibility tools for agencies & webmasters
Coblis - If you are not suffering from a color vision deficiency it is very hard to imagine how it looks...