Ask HN: Which email-me-when-my-site-is-down service do you use?

Recently my Debian 8 server started "being down" from time to time. It does not respond to http,ping and ssh. Nothing is in the logs. Just like the power has been turned off. Only when I reboot it, it comes back.

It's a dedicated server rented from 1and1.

While I try to find the problem or a new provider, I would like to get notified when it is down.

There must be an endless number of free services for this. Can my fellow startup buddies from HN recommend one?

I use monitive.com totally happy with them.