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.
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!
This is a great tool. I used it for monitoring my own network with 10 machines and about 170 checks. It was quicker than GFI Monitoring tools.
After a while we tried the NSCA plug-in to check our Sonic MQ container, brokers and queues. Worked great.
I am a happy user