Really Good UX - A library of screenshots for UX inspiration
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.
Usability.gov - Resource for user experience (UX) best practices & guides
LinkChecker - LinkChecker is a free, GPL licensed website validator.
UX Check - Easy heuristic evaluations on your website (chrome ext.)
Pulno - SEO Audit and Website Analysis. Pull up your SEO.