NUT, the Network UPS tools, consists of three small programs that run interdependently: the driver for the particular UPS, which understands the protocol it speaks, the upsd daemon, which communicates with the driver and responds to client requests, and the client software, which can reside locally or remotely. Having three separate tools provides great flexibility: several low-power devices can share one UPS and listen to the one daemon. One daemon can maintain communication with several UPS drivers. Clients are available on multiple platforms for interoperability. Elegant and simple design. Visit the home page at http://www.networkupstools.org/
UPDATE: A handy reference for installing and configuring NUT so that it automatically starts on bootup of OS X: http://www.llondel.org/ups.shtml