Passmarked is THE tool I've been looking for to check the quality of websites (mine & clients'). The open source list of tests makes it incredibly useful and flexible for integrating into the development workflow. It's only just been launched but there's already a tremendous amount of coverage that will only get better as more people contribute.
Dr. Link Check helps you track down broken and malicious links on your website. The service crawls through the entire code of your site and gives each discovered link a thorough examination to be sure the link works as intended and doesn’t point to a server known for hosting malware or phishing attacks.
AccessLint brings automated web accessibility testing into your development workflow. When a pull request is opened, AccessLint reviews the changes and comments with any new accessibility issues, giving you quick, timely, and targeted feedback, before code goes live.