What does your team use for uptime monitoring?

Backstory: I've been running an uptime monitoring service for over a year now, and I realised much of the value comes from what happens after we detect downtime (alerts, incident response, and incident communication).

So I'm looking to integrate as many monitoring services as possible so folks can automate their team's incident response as well as communicating what's happening to their customers.

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Hence the question: What does your team/employer use for uptime monitoring?