did you try a google search? or http://freshmeat.net
Look at the UPS readme http://www.tldp.org/HOWTO/UPS-HOWTO.html
and also check out NUT, Network UPS Tool, http://www.exploits.org/nut/.
There are other tools besides NUT, but I feel NUT is most popular.
There's plenty of documentation on the specifics of implementing the system.