A few days ago, I set out trying to use Growl to let myself know my battery level was getting low.
I was going to write a longer post about my solution, but this morning, somebody posted a hint at macosxhints.com this morning about using the command-line tool growlnotify.
My own setup also relies on growlnotify, so I thought the details of my script would work better as a comment on that hint than as a post on my own site.
Enjoy. There’s also an AppleScript alternative for those who don’t want to use Growl.
Don't Look Now 