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.
accessiBe - Making websites accessible to people with disabilities
tota11y - An accessibility visualization toolkit from Khan Academy
Page Speaker - Modern web accessibility tools for agencies & webmasters
Koa11y - Automatically detect accessibility (a11y) issues on webpages
Stark - Contrast check and color blindness simulator for Sketch.