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.
Majestic - Majestic - Link intelligence tools for SEO and Internet PR and Marketing. Site Explorer shows inbound link and site summary data.
UptimeRobot - Free Website Uptime Monitoring
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.