I’m on a mission to host my websites as cheaply as possible (currently via a cheap Ionos VPS & Porkbun), and I want to know what quality of service I’m getting. So I Googled “open-source uptime monitors”, and found this, which I started using yesterday:
The clever thing about it is that it’s entirely powered by GitHub Actions, and it uses GitHub Issues as an alerting mechanism. There’s an Action that seems to run every half an hour* that checks whether your sites are up, another that seems to run twice a day that measures the response time of each site and commits it to a file, and another that generates response time graphs once per day from the history of these response time files. It also generates a GitHub Pages site like this, to show the statuses of your sites.
I came across UptimeRobot too, which has a free plan, but I was so impressed by the novelty of Upptime running everything through GitHub Actions that I didn’t bother to check it out. 
Do any of you have experience with free uptime monitors?
* It’s actually supposed to check that your sites are up every 5 minutes, but GitHub doesn’t seem to have the capacity for this. The current monitoring frequency is fine for my low-traffic sites though.