OnlineOrNot - Uptime monitoring for software teams (even teams of one).
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.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
Uptime Kuma - A fancy self-hosted monitoring tool.
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.