Monitoria provides active monitoring for your website. It will periodically send request to the website and verify it's still running. When it detects an issue, it will send you a notification.
Uptime monitor will send requests to your website in short intervals and notifies you immediately when your website goes down.
SSL monitor will send a request every 24 hours and notifies you if your SSL certificate is close to expiration. You can configure how many days in advance you receive notifications.
UptimeRobot - Free Website Uptime Monitoring
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.
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
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Better Uptime - We call you when your website goes down