FluxBat is an ACPI based battery monitor for the fluxbox window manager. It includes two parts: a daemon process that runs the update script and the update script itself. The whole software contains only about 100 lines of code and is easy to use by including only one line in the menu configuration file of fluxbox.
MonAMI aims to be a standard place for monitoring the availability and performance of services. It can talk to many different monitoring systems while remaining easy to configure. It has a plugin structure, making it easy to add new things to monitor or to integrate with new monitoring systems.
Network UPS Tools (also known as NUT) is an extremely powerful and versatile client/server based approach to UPS monitoring. Products from a wide range of vendors (APC, Belkin, Best Power, MGE, Tripp-Lite, etc.) are supported. A shared UPS can be monitored by one host, notifcations being sent to the other hosts via authenticated TCP connections. A Web interface is available, as is a Windows client.