If you want to hand off all the hard stuff about monitoring and get some easy to use, core functionality (graphing, alerts, dashboards) then my company https://www.serverdensity.com has been going 7+ years now.

For highly sophisticated environments then https://www.datadog.com is a very advanced product.

Both are based off my original agent: https://github.com/serverdensity/sd-agent (DataDog forked it in 2010 and we forked them back last year).

We're also behind http://www.humanops.com trying to build monitoring that also helps you run on-call and ops teams generally in a way that considers fatigue, stress and the realities of the humans running IT systems! E.g. https://blog.serverdensity.com/introducing-alert-costs/ and https://opzzz.sh