Microsoft Network Monitor, Nagios, Fiddler, Observium and Ganglia are examples of network monitoring freeware. They are all highly recommended and each one offers different features to monitor various types of networks.
Microsoft Network Monitor is a packet analyzer that allows the user to capture, view and analyze network traffic. This tool is handy for troubleshooting network problems and applications on the network. Main features include support for over 300 public and Microsoft proprietary protocols, simultaneous capture sessions, a Wireless Monitor Mode and sniffing of promiscuous mode traffic.
Nagios is a powerful network monitoring tool that helps the user ensure that the critical systems, applications and services are always up and running. It provides features such as alerting, event handling and reporting. The user can set up email, SMS and printed paper alerts. Even though manual configuration might seem difficult, that setup actually makes Nagios one of the most flexible network monitors around.
Fiddler is a web debugging tool that captures HTTP traffic between chosen computers and the Internet. It allows the user to analyze incoming and outgoing data to monitor and modify requests and responses before they hit the browser. Fiddler gives extremely detailed information about HTTP traffic and can be used for testing the performance of websites or security testing of web applications.
Observium is an auto-discovering PHP/MySQL/SNMP-based network monitoring tool. It focuses on Linux, UNIX, Cisco, Juniper, Brocade and more. With Observium, the user finds detailed graphs and an easy-to-use interface. The tool can monitor a huge number of processes and systems. The only downside is a lack of auto alerts.