Nagios is an open source host, service and network monitoring program. It contains much (not all) of the functionality found in expensive commercial monitoring systems like HP Openview at a mere fraction of the cost.
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Nagios can monitor virtually any type of device from Windows desktops, to managed switches, to Linux servers. Notifications can be made via email, pager, SMS, etc.
With a legion of active developers, Nagios is rapidly growing in popularity in the networking world. So before you drop big money on a monitoring system, check out Nagios (http://www.nagios.org). While Nagios isn’t a full-blown SNMP monitoring platform, it has a surprisingly wide range of functionality. You may find that it fulfills all of your requirements and leaves a lot of change in your pocket!