It's not exactly a small cli tool as such, but I can heartily recommend Zenoss Core for network and system monitoring. It has restored my faith in monitoring-and-alerting software.
Where tools like Nagios, Cacti, etc. all have steep learning curves and seem to be good at only one thing, most often you have a need for an all-in-one solution, and Zenoss Core provides that for free (and Free).
It's hard to summarize, but basically it's an application with web-interface that autodiscovers devices on your network, then monitors tons of parameters, logfiles, ports, services, software, hardware, amount-of-pages-printed, etc. If anything fails or crosses a threshold you get an alert. It's easy to get started: enable/install snmp on each device you want to monitor, install Zenoss on a server, and open the web interface.
The free Core version is good enough for small and medium businesses, while the commercial Professional and Enterprise versions provide some advanced features.
If you ever thought about Nagios, Cacti or the like, be sure to evaluate Zenoss as well.