Free Open Source Software has a long tradition in providing system monitoring software. And this domain is still a stronghold for traditional programming languages of system administrators, shell and Perl.
This overview distinguish basically two different kind of Monitoring software: All purpose server monitoring and special device monitoring. Of course the boarders are blurred. While MRTG started as a special network device monitoring tool where the focus is to know what happened in the past. For example how much traffic has been seen last month. This is marked as ‘Usage’. The other solutions, like the Nagios orientated ones, focus more on the question if a certain server is up or not and if the data of certain devices are in expected ranges or not. This is marked as ‘Mon.’.