wmbattery displays the status of your laptop's battery in a small icon. This includes if it is plugged in, if the battery is charging, how many minutes of battery life remain, and battery status (high - green, low - yellow, or critical - red).
There's nothing in the program that makes it require WindowMaker, except maybe the look. It can be docked in WindowMaker or AfterStep's dock.
wmbattery can use APM, ACPI, or even the SPIC that is in some Sony laptops. With ACPI, it upports multi-battery machines, and can estimate how long it will take the battery to finish charging or discharging.