Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.
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.
Volument - A smarter take on website analytics
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Fathom Analytics - Simple, trustworthy website analytics (finally)
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.