Here's a snapshot of each:

• Nagios. A pure open source project. Can be downloaded separately or found in Linux distributions from Debian, Fedora, Suse Linux and OpenBSD Unix, as well as Groundwork Monitor (see below). Corbo says it's targeted at network engineers, requiring some technical expertise before being deployed system. Still, it has an extensive library of monitor plug-ins.

• OpenNMS (www.opennms.org). Free. Like Nagios, according to Corbo, it needs some work before being integrated into your environment. Support and consulting services can be bought through the North Carolina-based OpenNMS Group . Tarus Balog, that company's CEO, said the agentless OpenNMS can monitor "hundreds of thousands of devices." It ships with some 40 monitors and includes escalation alerts and data collection. The goal of Open NMS 1.2 is to compete against HP Network Node Manager, he said, while the upcoming version 1.5 aims to compete against IBM's NetCool.

• Zabbix. Free. Corbo notes its ease of installation makes it close to a turnkey software, which is its strength. Like Nagios, it can be found in several Linux distributions, including Debian and Ubuntu.

• Zenoss. CTO and co-founder Erik Dahl says the goal of the agentless Zenoss is to monitor the entire IT stack. While the Appliance can monitor up to 1,000 devices, the Enterprise software version can scale up to 20,000 devices. Dahl said the company has worked hard to make sure it plays well with Windows. Enterprise users also get ZenPacks, plug-ins that expand the system's capabilities. Corbo notes that Zenoss Core is full-features, but Enterprise has an enhanced user interface, reporting capabilities and better multiserver management features.

• GroundWork. Going beyond performance monitoring, IDC calls it a framework for Windows, Unix, Linux, Netware and IBM iSeries system management. According to David Dennis, the San Franciso company's senior director of product marketing, it integrates a number of open source tools, including Nagios. Comes in community, Professional (single location) and Enterprise (same features but supports thousands of devices) versions. Its default mode uses agentless monitoring, but for those who prefer there is an optional agent. Dennis said Monitor offers "on par" scalability with products from IBM and HP. GroundWork's strength, says Corbo, is its ability to let users create a custom monitoring system.

• Hyperic. Stacey Schneider, the company's senior marketing director, said Hyperic HQ (community) and Hyperic Enterprise versions manage over 70 application servers, databases and switches. The Enterprise version adds the ability to create templates for rules as well as high availability. Unlike others, which are priced by level of support, Hyperic is priced per CPU socket pair or network device. According to IDC, Hyperic's strength is in its specialized agents for reporting detailed service information.

These applications have varying capabilities, IDC notes. For example, while Hyperic, GroundWorks and Zenoss can some ability to restart and reboot some servers and services, these are not built in to Nagois or OpenNMS. Zenoss and GroundWorks can show if there's compliance with customer service level agreements, while OpenNMS, Zabbix and Hyperic keep historical availability reports.

Corbo adds another caution: Like all open source applications, just because a licence is free doesn't make the cost of ownership cheaper. Support and training and deployment costs have to be factored in.