I recently came across OpenNMS, a network management system, similar to Nagios. Their site is at http://www.opennms.org/index.php/Main_Page with a demo site that shows you how it works. From their web site:

OpenNMS is the world's first enterprise grade network management platform developed under the open source model. It consists of a community supported open-source project as well as a commercial services, training, and support organization.

In a similar vein is GroundWork's setup (http://www.groundworkopensource.com/) which includes a VMWare appliance download to get moving in a matter of minutes. GroundWork's product (based on what I have seen) is based on Nagios and a bunch of other projects. From GroundWork's site:

GroundWork Monitor is the most scalable open source IT monitoring solution for companies with heterogeneous operating systems, application and hardware environments who want to reduce ongoing monitoring costs, consolidate views and reports and improve staff productivity.

The best part? Its free and in freedom and free as in beer. No vendor lock-in (unless that kind of thing tickles your fancy), and no proprietary hand waving mumbo jumbo. Feel free to download and run installs of your own!