Cacti is a complete network graphing solution designed to harness the power of RRDTool's data storage and graphing functionality. Cacti provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features out of the box. All of this is wrapped in an intuitive, easy to use interface that makes sense for LAN-sized installations up to complex networks with hundreds of devices.
Nagios® is a system and network monitoring application. It watches hosts and services that you specify, alerting you when things go bad and when they get better. Nagios was originally designed to run under Linux, although it should work under most other unices as well.
Some of the many features of Nagios include:
A popular VMware appliance, Untangle protects against viruses, spyware, phishing, spam, and other threats and nuisances, bundling the likes of ClamAV, Snort, SpamAssassin, OpenVPN, iptables, and other open source goodies with its own scanning engine. Untangle combines everything into a single server, and includes updates to the applications, various signatures, filters, and category lists along with nice reporting.
Wireshark is the world's foremost network protocol analyzer, and is the de facto (and often de jure) standard across many industries and educational institutions.
Zenoss Core is an open-source network monitoring and server monitoring product. Currently the most popular IT Management project on SourceForge.Net, Zenoss Core source code is available for free downloads. Zenoss core features include: Network, Server, and Application monitoring with a single product, ability to monitor any SNMP enabled device, real-time alerting when outages or slowdowns occur and single Event Console containing Zenoss alerts, SNMP traps, and log events.