#5 Collectd:

Collectd is performance monitoring software that collects transfers and stores performance data of computers and network equipment. Developed by Florian Forster, Collectd was written as Unix Daemon which later changed itself into an open source project. Later developers kept on patching and writing extensions into the software. Now most files of the source code are licensed under the terms of the GNU General Public License, version 2 (GPLv2), while the remaining files are licensed under other open source.

#4 Observium:

Observium is a PHP and MySQL-based Network Observation and Monitoring System (NOMS) which collects data from devices using SNMP and presents it via a web interface. The software uses RRD tool for analysis. Some of features provided by Observium are minimum interaction, maximum automation and maximum accessibility of information. This monitoring system focuses primarily on Cisco and Linux networks but includes support for a wide range of network hardware and operating systems. The software also monitors resource breakouts within a network.