I'm trying to write a small app that monitors how much power is left in a notebook battery and I'd like to know which Win32 function I could use to accomplish that.
For Vista and up you can use RegisterPowerSettingNotification
For earlier functions see the Power Management Functions in this section of the MSDN page "Power Management Functions: Windows Server 2003 and Earlier"
You can see example code of the Vista method on codeproject.