I used to have NewRelic on https://doorbell.io for my monitoring. It worked pretty well for the basic things, and the basic plan is free.

However, as https://doorbell.io's stack got increasingly complicated, the plugins of NewRelic didn't work as well as I needed, in order to reliably monitor all aspects of the platform.

I decided to try out Scout as an alternative, since even though it doesn't have a free plan, the basic plan is only $8/month (compared to $149 for NewRelic).

I found the interface to be really good, and they have great documentation. I found plugins for every single part of my stack, and they all worked very easily "out of the box". And best of all, added practically no overhead to the server!

So overall, I'd say it's a service that's well worth paying for. It's a steal at $8/month!