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We use Opsview rather than Nagios. It's easier to install and configure, and supports distributed monitoring out-of-the-box. I also prefer the GUI over Zabbix, Groundwork and Nagios. Opsview's community version is also properly Open Source (GPLv2) which you can't say is the case with all of those listed. The other monitoring product I really like is Zenoss, although its not based on Nagios core.