Just convinced my mum to migrate to wp7. Unfortunately she only has 1 GB of data. So she needs a decent data monitoring app. My carrier doesn't have one for wp so need a non-carrier app. Not prepared to pay for it.
If it's a Nokia Lumia, try Nokia's free Counters app.
+1 to that! I'd given up trying to find a usage monitor but stumbled across 'Counters' and it's fine for what I want. A bit basic when compared to some of the Android ones I've used in the past but it does the job