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.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Monkey Test It - Primate powered algorithms that scan your site for common bugs and SEO mistakes.
Better Uptime - We call you when your website goes down
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.