It requires some setup but the big advantage of this is that it can not only tell you when your site is down, but sometimes ACTUALLY FIX IT by restarting nginx or whatever you need it to do.
This is something a monitoring service can't do since they don't have access to your box. Of course it's not going to be able to fix all situations that could cause downtime, but if it hits a scenario you anticipated it will.
Don't use it to the exclusion of external tools though. monit doesn't help if the server has no connectivity or no power. On a similar tangent, configuring a mutual restart policy (eg: if cron fails monit restarts it and vice versa) is also a good idea.
Actually automatic countermeasures are almost always a bad idea. Don't do it.
If a service crashes repeatedly then fix the service, don't deploy bandaids. Those bandaids tend to pile up and god help you when that pile comes crashing down...
It requires some setup but the big advantage of this is that it can not only tell you when your site is down, but sometimes ACTUALLY FIX IT by restarting nginx or whatever you need it to do.
This is something a monitoring service can't do since they don't have access to your box. Of course it's not going to be able to fix all situations that could cause downtime, but if it hits a scenario you anticipated it will.