Threshold makes checking your data just an app touch away, and does a little more than that. You can even define alerts for the phone to remind you whenever you use a certain amount of data. These warnings will help remind you to use less data if you’re reaching your limit. The app can also keep track of all your data usage, monitoring “phantom data charges” so you know which apps are using data even when you’re not using your phone.
Threshold is available now on the Apple App Store for $3.99. A small price to pay to keep you from overrunning your monthly data limit and paying a much larger penalty.