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.
LinkChecker - LinkChecker is a free, GPL licensed website validator.
Whatsmyserp - Powerful Rank & Serp Tracking. Completely Free.
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
Backlink Tool - A backlink tool to index your links quickly and freely.
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.