LinkChecker - LinkChecker is a free, GPL licensed website validator.
SE Ranking - All-in-One cloud-based SEO software for agencies, marketing teams and webmasters
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
Linkody - Save time and money by managing your SEO linkbuilding with Linkody.
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.
Ahrefs - Ahrefs is a toolset for SEO and marketing. We have tools for backlink research, organic traffic research, keyword research, content marketing & more. Give Ahrefs a try!