Package:APC UPS Daemon
What is apcupsd?
"Apcupsd can be used for power mangement and controlling most of APC's UPS models on Unix and Windows machines. Apcupsd works with most of APC's Smart-UPS models as well as most simple signalling models such a Back-UPS, and BackUPS-Office. During a power failure, apcupsd will inform the users about the power failure and that a shutdown may occur. If power is not restored, a system shutdown will follow when the battery is exhausted, a timeout (seconds) expires, or runtime expires based on internal APC calculations determined by power consumption rates. Apcupsd is licensed under the GPL version 2." 
First, emerge sys-power/acpupsd (package not on wiki - please add):
# emerge apcupsd # equery f apcupsd
Configuration and Runlevel Registration
Edit your /etc/apcupsd/apcupsd.conf properly.
To start apcupsd, run the following:
# /etc/init.d/apcupsd start
To add apcupsd to the appropriate runlevels, execute the following commands:
# rc-update add apcupsd default # rc-update add apcupsd.powerfail shutdown
Check UPS status
Finally, to check on the status of acpupsd, run the following:
# apcaccess status # tail -f /var/log/apcupsd.events