Is It Down Right Now? - Network & Admin
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
Site Down - Site down outage and downtime reports from SiteDown.co
SEO Site Checkup - Find comprehensive search engine optimization (SEO) tools for your site.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
Sitechecker - All-in-one SEO platform including rank tracker, website SEO audit tool and website changes monitoring. Detailed reports by 100+ parameters with how-to-fix guides.