Nagios is a great tool for server monitoring, providing rich environment to setup different notifications and monitoring scenarios.

It supports hosts and notification service grouping, comes with a GUI out-of-the-box and has really great community and lots of documentation online.

Pretty much all the setup can be done, based on the google’s top search results. Nice place to start is Digital Ocean post about setting up Nagios on Ubuntu 14.04. There is  also another nice post, explaining how configuration is done.

If you fail to install package, or your server is different from Ubuntu, f.e, you run some Debian Stable server, you can address this post (RU) and build Nagios from source.

Configuring client can be done via this post, but check_nrpe command available only on server, not on the client.