Sysmon when reading Open Source Network Administration.
Once I wrote my own configuration file to watch my systems, I followed the book's instructions to complete the Sysmon installation. The result is the small screen shot at left. Since you don't need to spend a lot of time checking out the details of my network, you can get the idea by reviewing this image. The green systems are all up. The yellow ones just became unreachable. I forgot to return on one of them to service after working on it. Since it's a bridging firewall for the second box, both are listed in yellow. The orange/reddish entry is a missing box. I lent it to my office for a case, and when it returns the record will be green again!
Sysmon is a good alternative to Nagios or the Network Management Information System (NMIS) if you only need to do simple monitoring of 10-100 systems. Sysmon can check availability by reading HTTP replies and connecting to arbitrary TCP ports, as well as pinging remote systems.