-
Uptimerobot: https://uptimerobot.com/
-
updown: https://updown.io/
The first one has a mobile app and sends notifications via app and email in case of abnormalities.
The free version only allows monitoring for 5 minutes.
The second one is the one I've been using recently and it also sends email notifications.
It allows monitoring every 30 seconds and allows custom monitoring routes.