OpenNMS has had its ups and downs between versions, but has always been reliable and free monitoring. There are a lot of features that can be enabled with OpenNMS through the browser, but some will require having to open a text editor and manually creating or editing a text file. While there is a learning curve to this monitoring system if you've had experience with other monitoring systems, there is a very large support group that is always helpful.
Easy to use after configuration.
Supports escalations and different contact groups.
Not the easiest to configure for multiple groups supported.