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.
Xenu's Link Sleuth - Xenus Link Sleuth checks Web sites for broken links.
tota11y - An accessibility visualization toolkit from Khan Academy
Scrutiny - Scrutiny for Mac helps you manage and improve any number of websites. Link check, search, full website spell check, XML sitemap generator, SEO audit
Accessible Metrics - Digital Governance
LinkChecker - LinkChecker is a free, GPL licensed website validator.