StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UptimeRobot - Free Website Uptime Monitoring
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.
Uptime Kuma - A fancy self-hosted monitoring tool.
UptimeBar - A simple OS X menu bar app for monitoring website uptime
OnlineOrNot - Uptime monitoring for software teams (even teams of one).