I am a fan of open source tools. The Internet, as we know it today, will not exist if not for the open source movement. We owe this to the countless architects and developers who dedicated their time and effort to write open source software.
Enterprise IT departments can also take advantage of open source software. Numerous companies have been using them for years. One particular area where they can be used is network and server monitoring.
There are a lot of open source network monitoring tools out there. Leading the pack are Nagios, Zabbix, and Cacti. My favorite tool though is OpenNMS. I particularly like it because it is very easy to setup and administer. It can automatically discover your nodes on the network. There were very few tweaks when I first set it up. It provides simple event and notification via email or pager. In addition, its web-based management interface interface is very easy to use.
I have been using OpenNMS for several years now and it has been running rock solid. I definitely recommend OpenNMS for IT departments who do not want to pay a hefty price to monitor their network and servers.