We use Pandora FMS to monitoring all our systems: hosts Unix, windows, Macs, routers, swithches, our web page, all our services, some temperature sensors... It's a very flexible tool, and easier than other tools to manage. Before, I used Nagios and when I had to monitor any special data, I had to write a new plugin and I'm not a developer, it was very difficult to me.
If you are by testing tools, try Pandora FMS