I've just bought a new APC UPS for my PowerMac G4. It ships with PowerChute software, but versiontracker.com comments suggest it is not yet Panther compatible. It seems, however that Apple has included UPS software in Mac OS X, they just forgot to mention it.
By opening Battery.menu in the folder System -> Library -> CoreServices -> Menu Extras, you will get a new menu indicating battery level and remaining time if you are running off your battery. I haven't had the nerve yet to see if my Mac will automatically shutdown if the battery level goes too low yet, but I don't see why it shouldn't (anyone want to try?).
This will only work if your Mac is running 10.3 and is plugged in to a compatible USB UPS. Jaguar seems to have some support for UPSes, but not in the Battery menu extra.
[robg adds: I don't have a UPS with which to test this one ... can anyone else confirm this behavior?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040203154052326