Autostatus is a network monitoring program for hosts and services. It is designed to be
fast and extremely easy to use. It monitors by icmp pings and by
attempting TCP connections to specified ports.
Thanks for Autostatus, it's excellent!
Still using Autostatus after 12 years, despite having a Nagios system as well. The core algorithm used by Autostatus is still the most efficient and "correct" system for monitoring network equipment availability--taking into account the dependency tree and checking things in the correct order.