Opsview is by far the most flexible monitoring solution I have ever worked with.
It uses some unique features like autodiscovery and host-templates to make configuring a host really easy.
By deploying keywords one can make unique selections within Opsview which can be used to create specialist views and for various reports on availability, performance and SLA's.
It is highly scalable using slaves and slave clusters.
The Opsview Dashboards allow for advanced visualization of the IT estate monitored by Opsview.
The REST API allows for automation on any level (adding/deleting hosts and so on).
And finally the Opsview Mobile app allows any user to stay in touch with the system at all times.
UPDATE: recently Opsview 4.5 has been released which has proven again to be a step forward in Opsview's commitment to be the best monitoring solution available.
Included in this new version is the Business Service Monitoring feature.
BSM allows users to create groups of components based on their "business role" (ie. your public website consisting of webservers, appication servers and database servers for instance).
BSM takes into acocunt things like redundancy, availability and so on.