What is your go-to free tool for tracking site uptime and server errors?

The worst feeling is when you wake up and your site was down for 6 hours and you don't even know. UptimeRobot is the popular one. The free plan would help you check every 5 minutes and send email to that effect. Some people use Pingdom but the free version is limited. Others just dey manually check their site every morning like a ritual. How do you do yours?