I have recently started using a piece of software called Paessler PRTG. This is a network monitoring tool which keeps tabs on your network devices and services and can alert you or take action if it detects any problems. I originally went looking for some software to monitor my broadband connection, which has a tendency to come and go. After a bit of googling, I came across PRTG which seemed to be able to do the job, and had the appeal of a freeware version which was limited to 10 "sensors". Not knowing what a sensor was, I wasn't sure if this constraint was going to bite, but the first 30 days is unlimited anyway, so I downloaded and installed.
Installation of the software was a speedy process, though PRTG then sets about "auto-discovery" to identify all the devices and services on your network and create the corresponding sensors it will use to monitor those devices. So, for example, I have a number of Sonos (wireless audio) devices. PRTG found these and added a simple Ping sensor for each device. The Ping sensor pings the device periodically, and records the response time. If it gets no response (or another error), the sensor goes into "alarm". But for other devices (eg my Windows PC, my router, my Apple time machine), PRTG can set up numerous sensors which monitor different aspects do the device's performance/status. It recognised that my router provided my Internet connection, and established a sensor to monitor the time taken for a DNS query. I use this to check whether my broadband connection is working. As a rough guide, PRTG found about 80 sensors for my home network.
The software is very easy to use, and has loads of nice features (such as emailing you with todo's for software updates!). The display is very easy to read and the graphs showing availability, up-time etc are really useful. For example, I soon discovered that my Apple TV, which is connected by Ethernet, was not responding intermittently - having this evidence gave me great confidence in getting Apple to replace it.
Check it out at http://www.paessler.com/prtg.