Observium is an autodiscovering network monitoring system focused primarily on Cisco and Linux networks but includes support for a wide range of network hardware and operating systems. Observium has grown out of a lack of easy to use NMSes. It is intended to provide a more navigable interface to the health and performance of your network. Its design goals include collecting as much historical data about devices as possible, being completely autodiscovered with little or no manual intervention, and having a very intuitive interface.
LibreNMS is an autodiscovering PHP/MySQL-based network monitoring system forked from Observium. It aims to be easy to use and painless to deploy, and to support monitoring of a wide range of devices. Observium's license changed to a modified QPL license in May 2012, then was split into free and commercial versions. The aim of LibreNMS is to continue to create a usable NMS, but with a community focus and GPL licensing.
aerotools-ng provides a C interface for accessing the Aquaero5 USB device by Aqua Computer. It consists of a set of functions for data access and conversion, and a commandline tool using these functions as an example implementation. It also includes utilities for exporting sensor data to SNMP to be used by NMS and other monitoring/graphing tools.
Sagan can alert you when events are occurring in your syslogs that need your attention right away. It can store events into a Snort database, so your IDS/IPS data and log data are in the same place. This enables a single console, like Snorby or BASE, to view not only your IDS/IPS data but your log (syslog, SNMP, etc.) data as well. Sagan will correlate the data for you. It also uses 'Snort-like' rule sets, which means it is compatible with Snort rule set management software. It supports multiple output formats that any network administrator will find useful. Sagan can also stop threats based on log analysis via "Snortsam". This allows Sagan to communicate with various types of network devices (Cisco routers/ASA/etc., Linux iptables, etc).