Network monitoring is critical if you want to know that your computer systems are working.
I looked into a few choices:
SmokePing: pings a bunch of addresses and graphs the response times http://oss.oetiker.ch/smokeping/
Nagios: Nagios is a free, full blown network monitor for Linux. When I looked into it, it was a fair amount of work to set up – there are a few books available
Another app is Groudwork opensouce edition.
I am always leary of open source software that has a paid version. Seems like what could be a great free app usually ends up being little more than a demo for the companies paid version.
Another tool I found is ServerSentinel (http://www.jam-software.com/serversentinel/index.shtml) Server Sentinel isn’t free, but it’s not pricey either $65. Has a nice variety of sensors (you can do a lot more than just ping an address)