We use it on most of our hosts to watch for busted processes/etc, and then I have a script that posts to slack if something goes wrong. I’m pretty sure you can run it on your own server to monitor external hosts if you want… https://mmonit.com/monit/
Another vote here for https://uptimerobot.com - it misses short incidents due to a 5 minute check interval on free accounts, but it’s good enough for me.
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https://www.val.town/v/stevekrouse/uptime
Spams you with a bit of ads. But free and works.