Just enter the data limit of your plan and the first day of the billing period, and the app will notify you automatically when you reach important levels of your data limit (e.g. 90%, 100%). Stay within your limit and never pay extra for exceeding it! The app will automatically start when you reboot your device, and it will work in background checking your data usage every 10 minutes and showing alerts when you reach certain levels (work in background is available on iPhone 3Gs, iPhone 4, iPad and iPad 2). It will also show % of the data limit you've already consumed on the app icon!
In addition, this application allows you to create and operate any number of "counters," which are an advanced way of tracking traffic (data usage) beginning with a particular moment of time. Several counters can function simultaneously, be set on pause, or reset entirely and allow you to compute the amount of traffic consumed by a particular application or the traffic consumed at a given location.
You can view the full content of that tour here.
You will find this program very useful if you use YouTube, Gmail, Yahoo, Twitter or Flickr.