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.
Free SEO Checker by Spotibo - An SEO checker that analyses based on Google's algorithm 👀
LinkChecker - LinkChecker is a free, GPL licensed website validator.
SEO Analyzer By Neil Patel - A free SEO analyzer for your site
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
SEO Minion - Chrome extension & Firefox add-on to help you with SEO