LinkChecker - LinkChecker is a free, GPL licensed website validator.
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.
DRKSpiderJava - DRKSpider is an open source website crawler (sitemap generator) and link checker. Features:
Dead Link Checker - Check for broken links on any website
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
Dr. Link Check - Broken link checker with malware and parked domain detection