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.
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
Accessible Brand Colors - A tool to make sure your brand colors are ADA compliant.
whatdevsneed - This is whatdevsneed.
Page Speaker - Modern web accessibility tools for agencies & webmasters
SmallDevTools - Handy developer tools with a delightful interface