Both the commercial and open source worlds are crammed to the gills with network monitoring software and appliances. Are you paying a high price to get a true enterprise-worthy network monitoring system?
You need not, for once again the open source world comes to the rescue with OpenNMS. OpenNMS is a pure SNMP network monitoring tool. This means that anything SNMP can do, OpenNMS can do: service polling, data collection, and sending alerts when troubles are detected. OpenNMS scales nicely, from tiny installations monitoring several dozen hosts to huge globe-spanning networks containing tens of thousands of nodes.
Installing OpenNMS is a bit hair-raising due to its many dependencies. So far as I know, there is no easy prefab "appliance"-type installation bundle. Once it is up and running it does a lot of things on its own, such as network discovery, polling and notifications. The default installation supports 25 services. As you add services and hosts, OpenNMS automatically finds and monitors them.
OpenNMS is considerate of network bandwidth. It polls at five-minute intervals during normal operations. When it finds an outage, it increases the polling interval to 30 seconds and sends alerts. If the problem persists beyond five minutes, OpenNMS goes back to a five-minute polling schedule. These intervals, of course, are configurable.
OpenNMS' configuration files are XML-based, which can be either daunting or pleasing, depending on how you feel about XML. They are plain-text, which is always good because it means you can study and learn your way around them. A fair number of operations are controlled from the Web-based administration GUI.
Commercial support and training is available. In fact, you could take advantage of their training, become an OpenNMS guru your own self, and go into business. The OpenNMS team is committed to keeping OpenNMS licensed under the GPL and not using the dual-license dodge (i.e., where the open-source free-of-cost version is dumbed-down and missing features and all the cool stuff goes into the semi-closed commercial version). There are only two releases: stable and development. Source code is readily available as a free download. Visit OpenNMS.org for more information.