LinkChecker - LinkChecker is a free, GPL licensed website validator.
Ahrefs Backlink Checker - With the world's biggest index of live backlinks (over 15 trillion), Ahrefs Backlink Checker will show you the most complete backlink profile for any website.
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
Backlinks by Neil Patel - A backlinks research tool for competitive intelligence
Dead Link Checker - Check for broken links on any website
SEMScoop - Find and analyze easy to rank long-tail keywords with free SEMScoop Keyword Difficulty Tool, Start using them for SEO, PPC, content creation and all your marketing activities