SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
SEO Site Checkup - Find comprehensive search engine optimization (SEO) tools for your site.
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.
Majestic - Majestic - Link intelligence tools for SEO and Internet PR and Marketing. Site Explorer shows inbound link and site summary data.
SEOquake - SEOquake provides a browser extension to conduct on-page SEO audit.
SEOmator - SEO website analysis & audit tool