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.
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.
UptimeRobot - Free Website Uptime Monitoring
SEO PowerSuite - Get all SEO tools in one pack - download free edition of SEO PowerSuite and get top 10 rankings for your site on Google and other search engines!
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UpSeo - UpSeo is a best-in-class website traffic generator, allowing you to get more potential visitors to your website and enhance conversions.