What is the best open source network monitoring tool/software ?

Example program Opsview

wireshark for protocol monitoring in the network

opmanager for server and network monitoring

I'll vote for Nagios which is enterprise level tool and worth to work on it.

I prefer Nagios this is a very good Monitoring tool

Nagios is "the industry standard in IT infrastructure monitoring,"–well, at least that’s what it says on their website. Nagios can be complicated to install and configure, but its wealth of features are unmatched by any tool out in the market and is geared for the experienced IT network administrator. Nagios supports monitoring of multiple hosts and can send out alerts via email, pager (if you still use this ancient technology) or SMS/text messaging. Like Monit, it can also be configured to automatically respond to problems.

Nagios

in my opinion Nagios is the best open source network monitoring tool

OpenNMS, ZenOSS, Nagios, Cacti, MRTG & JFFNMS

I'll go with Nagios, Cacti and MRTG per the sequence. However at higher level and with additional tools, the vendors will definitely charge you.

I have to disagree with the majority. I'd go for THE DUDE by Mikrotik.

OpenNMS is an open source enterprise

Nagios is the old guard of system and network monitoring. It is fast, reliable, and extremely customizable. Nagios can be a challenge for newcomers, but the rather complex configuration is also its strength, as it can be adapted to just about any monitoring task. What it may lack in looks it makes up for in power and reliability.