Icinga is an open source network monitoring platform. It includes automation, modularized integration packages, and prebuilt alerts and reporting capabilities.
Icinga is a world-class monitoring system. It can be used for most general monitoring situations. It is not a silver bullet, however, and there are instances where domain-specific monitoring systems are necessary. However, the output from those monitoring systems can be funneled into Icinga as a central monitoring and alerting system.
Icinga is a solid solution which does everything it promises. It is backwards compatible with most Nagios instances, making the transition very easy. Once you get the hang of installing new plugins and editing configuration files expanding its monitoring capabilities are easy.
Icinga is better than Nagios because of its nicer user interface. New Relic can monitor CPU/memory and disk usage, but it's more of a performance and application troubleshooting tool rather than monitoring
I haven't used other products that are similar to Observium. Observium is an open tool, with a robust feature set. There are elements of comparable monitoring across other tools, but nothing I've seen that is a complete parallel in terms of feature set and visualizations. If I were asked, I wouldn't have another tool to recommend over Observium.