SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
Dr. Link Check - Broken link checker with malware and parked domain detection
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.
BrokenLinkCheck - Free Broken Link Checker / Online Dead Link Checking Tool - find bad hyperlinks
SEOquake - SEOquake provides a browser extension to conduct on-page SEO audit.
LinkChecker - LinkChecker is a free, GPL licensed website validator.