UptimeRobot is the world’s leading uptime monitoring service. UptimeRobot checks your service in 1 min intervals (5 min in case of free plan) and you get notified about every outage.

You can monitor
WEB: check any website (http/https)
SSL: get notified when SSL certificate is expiring
PORT: monitor any port, e.g SMTP, FTP, DNS or custom
PING: just ping (ICMP) if server is responding
KEYWORD: check if keyword is present or missing on a page
HEARTBEAT: monitor scheduled jobs (i.e. cron) – Not available in free plan

Feature:
• Overall status screen to check if any of your service is down
• Up & down events history
• Monitor list with handy search and filter features
• Monitor detail with detailed uptime, response time and event history
• Creating, editing, pausing and deleting monitors
• Push alerts about the up & down events
• Color-blind accessibility
• Third-party integrations like Slack, PagerDuty and many more (available only in the web app)

Free plan offer 50 monitors.

Web: https://uptimerobot.com