The second most popular application you'll see used to monitor PostgreSQL servers is Cacti. Cacti relies heavily on web application technology and RRDTool to do its work. This approach, heavily leveraging external libraries but including more features in the core, gives working with Cacti a slightly different feel than using Nagios, from installation to usage.
Cacti is primarily a trending system, and it doesn't do alerting. It generally has a nicer UI and is more powerful for analysis purposes than Nagios. But its scope is limited. Where Nagios aims to satisfy all sorts of purposes moderately well, Cacti focuses on the monitoring portions that require trend analysis (the strength of its RRDTool core), trying to do a superior job in ...
