Having been a Mac OSX user now for some years I appreciate the simplicity of CoconutBattery (http://www.coconut-flavour.com/), which shows you all relevant information of the battery of your laptop (iBook, PowerBook, MacBook, ...) and has one outstanding feature: you can save the current health state of the laptop's battery and thus monitor the long term development (degradation) of the battery. Since I have to work in Windows at work :-( I was looking for a similar tool in the MS world. You can find a number of battery monitoring programs for MS Windows, which give you a plethora of (useless) information - or do not work at all. Finally I decided to make one of my own (the world waited for that so badly!). And here it is.
Since I did not want to restrict it to Windows only I made it a GTK application, which is cross platform portable. I created the low level data gathering functions for both Windows and Linux. You are very welcome to add additional ports (e.g. BSD or whatever).