Is It Down Right Now? - Network & Admin
SEO Site Checkup - Find comprehensive search engine optimization (SEO) tools for your site.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
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.