Nagios can be installed on a server with very low specifications, our main server has 512 MB RAM and 1 processor.As the open source version does not provide a configuration interface, for those who need easy administration of Hosts and services - it can be integrated into other web applications such as NagiosQL (Personal favourite) with a database back-end to store configuration in the case of failure.There are many plugins we use on our desktops such as Nagstatus and Nagstatmon which provide us with real time monitoring results instead of sending out E-Mail alerts, Instant messages etc.We currently monitor all our servers using SNMP just to avoid installing apps like NSClient to keep monitoring simple and clean.
Room for Improvement:
The Installation can get very difficult depending on what Linux flavour you are using.
The open source version does not provide a configuration web interface. Some scripting knowledge is required in order to make changes to configuration files.
Nagios is my favourite open source monitoring system. I just love it!
It took me a while to properly configure Nagios to monitor our internal systems. Once we realised its potential we decided to setup remote polling on client systems, to have a central location for monitoring. (Using Centreon)As it is open source we were able to customise everything, from check commands, modifying the web interfaces, integrating into CMDBs such as iTop. When comparing Nagios to other monitoring systems it stands out by far.
If you are new to Nagios I recommend that you watch videos/read documentation before you start to mess around with the code. One little mistake can stop Nagios from monitoring your systems, so make sure that you always backup your configuration!