This is a collection of useful tools for accessibility testing and debugging. The earlier in the process you start using these, the better.

See also the accessibility section of my frontend-dev-bookmarks which contains many more resources.

• Accessibility Inspector for Mac: https://developer.apple.com/library/mac/documentation/Accessibility/Conceptual/AccessibilityMacOSX/OSXAXTestingApps.html
• VoiceOver for Mac: http://www.apple.com/accessibility/osx/voiceover/

• The Accessibility Viewer (aViewer) for Windows: http://www.paciellogroup.com/resources/aviewer/
• NVDA for Windows: http://www.nvaccess.org/download/
• JAWS for Windows: http://www.freedomscientific.com/Downloads/JAWS
• Up and Running with JAWS: https://joe-watkins.io/accessibility/up-and-running-with-jaws-on-a-mac/

• Diagnostic.css: http://www.karlgroves.com/2013/09/07/diagnostic-css-super-quick-web-accessibility-testing/
• debugCSS: http://yahoo.github.io/debugCSS/
• AcceCSS Sass mixin: http://lukyvj.github.io/accecss/

• The Accessibility DevTools extension for Chrome: https://chrome.google.com/webstore/detail/accessibility-developer-t/fpkknkljclfencbdbgkenhalefipecmb
• WAVE Evaluation Tool for Chrome: https://chrome.google.com/webstore/detail/wave-evaluation-tool/jbbplnpkjmmeebjpijfedlgcdilocofh
• ChromeVox for Chrome: https://chrome.google.com/webstore/detail/chromevox/kgejglhpjiefppelpmljglcjbhoiplfn
• aXe for Chrome: https://chrome.google.com/webstore/detail/axe/lhdoppojpmngadmnindnejefpokejbdd
• Color Contrast Analyzer for Chrome: https://chrome.google.com/webstore/detail/color-contrast-analyzer/dagdlcijhfbmgkjokkjicnnfimlebcll/related
• Siteimprove Accessibility Checker for Chrome: https://chrome.google.com/webstore/detail/siteimprove-accessibility/efcfolpjihicnikpmhnmphjhhpiclljc/
• TRAY Readability tool: https://chrome.google.com/webstore/detail/tray-readability-tool/eccbjfaplogblgjpopbihpgfgmlgjamf
• Funkify - disability simulator: http://www.funkify.org/

• Accessibility Evaluation Toolbar for Firefox: https://addons.mozilla.org/en-US/firefox/addon/accessibility-evaluation-toolb/
• DOM Inspector for Firefox: https://addons.mozilla.org/en-US/firefox/addon/dom-inspector-6622/
• Juicy Studio Accessibility Toolbar for Firefox: https://addons.mozilla.org/en-US/firefox/addon/juicy-studio-accessibility-too/
• WAVE Accessibility Extension for Firefox: https://addons.mozilla.org/en-US/firefox/addon/wave-accessibility-tool/

You can view a dump of the accessibility tree in Chrome by typing chrome://accessibility/ in the address bar.

• Tenon: https://tenon.io/
• HTML_CodeSniffer: http://www.squizlabs.com/general/html-codesniffer
• AccessLint: http://www.accesslint.com/

• Contrast Ratio: http://leaverou.github.io/contrast-ratio/
• Color Contrast Checker: http://webaim.org/resources/contrastchecker/
• Color Safe: http://colorsafe.co/

• aXe, accessibility engine for automated Web UI testing: https://github.com/dequelabs/axe-core
• tota11y, an accessibility visualization toolkit: http://khan.github.io/tota11y/
• Pa11y: http://pa11y.org/
• accessibilityjs - accessibility error scanner: https://github.com/github/accessibilityjs

• The Visual ARIA Bookmarklet: http://whatsock.com/training/matrices/visual-aria.htm
• Revenge.css: http://heydonworks.com/revenge_css_bookmarklet/
• Bookmarklets for Accessibility Testing: http://pauljadam.com/bookmarklets/
• a11y.css: http://ffoodd.github.io/a11y.css/

• Accessibility Testing Tools: http://www.paciellogroup.com/blog/2014/10/accessibility-testing-tools-updated/
• Screen Reader User Survey 6 Results: http://webaim.org/projects/screenreadersurvey6/
• Web Accessibility Evaluation Tools List (by W3C): https://www.w3.org/WAI/ER/tools/
• Easy Checks - A First Review of Web Accessibility: https://www.w3.org/WAI/eval/preliminary