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.
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
Accessible Brand Colors - A tool to make sure your brand colors are ADA compliant.
FreeStylerWs - Styles and skins for all popular sites in the web
Page Speaker - Modern web accessibility tools for agencies & webmasters
Amino Editor - Amino is a Chrome browser extension for customizing web page presentation with user CSS.