Today Nokia released Counters beta for Lumia Windows Phone devices. The Counters app lets you manage your calls, messaging and data use each month. You can even the option to reset your counters on your monthly billing date. This app is similar to the DataSense feature that will be part of Windows Phone 8, but with support for calls and messages as opposed to data alone.

Known issues:

there are no notifications when set limit is achieved (will come later)

conference calls are not counted properly

not possible to set data limit counter with a fractional part (i.e. not possible to set 1.5GB limit)

Download the app here from your Lumia Windows Phone device. Give your feedback about the app at Betalabs.

Thanks Jens for the heads up.