There is support for basic UPS monitoring at a very low level in Linux.  If 
you can get a serial port connection to take a line low when it's time to 
shut the system down, you've got what you need.  I believe there's support 
for "on battery", "Low battery", "battery out, shutdown", and "power 
restored" in the basic SysV initscripts and in the kernel or a low-level 
deamon.

Beyond that, if you're after fancy graphs of the battery's lifecycle-status or 
the average line voltage this week, you'll have to rely on a software package 
from the manufacturer.

Me, I've got this ancient box that has a mini-stereo plug that signals all I 
need to know.