Nagios you magnificent bastard.

Nagios is dead since they commercialized.
Icinga is the way to go.
Start out with Icinga 1, which supports nagios configs. In the meantime you migrate on by one to Icinga2. The configuration is completely different but it's worth converting.
Also still supports all nagios Plugins and the nrpe agent.
There is also a module named Icinga Director which could help with the configuration if you don't want to write from hand

@Kimmax
Going to check this out.

@ragnarr023 use Icingaweb2 as frontend, it's both beautiful, featuring rich (offers a full blown rest api) and is compatible with Icinga and Icinga2 via any DB Ido