SEO Analyzer By Neil Patel - A free SEO analyzer for your site
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
Serpstat - Search analytics, Rank tracking, Backlink analysis and Site Audit tool.
LinkChecker - LinkChecker is a free, GPL licensed website validator.
Serpstat Website SEO Checker - Check your website's SEO, analyze competitors and keywords
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.