I think you're referring to this:
http://forum.xda-developers.com/show...hlight=battery
It's very small (both in physical size on screen and file size) and effective, with the color range from green (full) to red (empty). I've been using it for months and love it.
P.S. You need to be logged in to that site to see/download the file, so I'll also upload it here as well.