accessiBe - Making websites accessible to people with disabilities
Koa11y - Automatically detect accessibility (a11y) issues on webpages
Page Speaker - Modern web accessibility tools for agencies & webmasters
EIII Checker Suite - The checker services can help you to locate and remove barriers on webpages or in PDF-documents.
tota11y - An accessibility visualization toolkit from Khan Academy
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.