Is It Down Right Now? - Network & Admin
Ahrefs Backlink Checker - With the world's biggest index of live backlinks (over 15 trillion), Ahrefs Backlink Checker will show you the most complete backlink profile for any website.
Site Down - Site down outage and downtime reports from SiteDown.co
Plerdy - Plerdy is a comprehensive conversation rate optimization software that caters to professional online marketers and agencies, allow them to enhance website performance and increase revenue greatly.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
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.