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.
Uptime.com - Everything you require for unified availability monitoring Simple & intuitive industry leading Enterprise-grade features delivered at a fair price, that are continuously improving.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Site-watcher.com - Specializing in website uptime monitoring. Receive instant alerts when your site is down or slow.
Better Uptime - We call you when your website goes down
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