SEOmator - SEO website analysis & audit tool
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.
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
UptimeRobot - Free Website Uptime Monitoring
SEOquake - SEOquake provides a browser extension to conduct on-page SEO audit.
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue