SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Serpstat - Search analytics, Rank tracking, Backlink analysis and Site Audit tool.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.
Better Uptime - We call you when your website goes down