Site Down - Site down outage and downtime reports from SiteDown.co
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.
Is it up? - A simple tool to check if a website or ip address is up or down.
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.
downinspector.com - Find out if a website is down for everyone or just you.
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.