ciao checks HTTP(S) URL endpoints for a HTTP status code (or errors on the lower TCP stack) and sends an e-mail on status change.

It uses Cron syntax to schedule the checks and comes along with a Web UI and a RESTfull JSON API.

Real-time performance monitoring, done right! https://my-netdata.io/

Glances is a cross-platform system monitoring tool written in Python. Glances an Eye on your system. A top/htop alternative for GNU/Linux, BSD, Mac OS and Windows operating systems.
https://github.com/nicolargo/glances

Get alerted when services go down or metrics go crazy

Self-hosted, easily-deployable monitoring and alerts service - like a lightweight PagerDuty