LinkChecker - LinkChecker is a free, GPL licensed website validator.
accessiBe - Making websites accessible to people with disabilities
DRKSpiderJava - DRKSpider is an open source website crawler (sitemap generator) and link checker. Features:
Page Speaker - Modern web accessibility tools for agencies & webmasters
Checkbot - Checkbot will boost the SEO, speed and security of your site by checking you're following 50+ web best practices. Browser extension with free and paid plans.
Koa11y - Automatically detect accessibility (a11y) issues on webpages