Free App: Data Usage [iOS] iTunes

Free today with ‘App of the Day’.
Take back control of your data usage. It’s time to stop paying overages.
• Monitor Mobile Data and Wi-Fi in real time.
• Today Widget
• Predictive forecast tells when you will go over and sets daily quotas.
• Simple and intuitive design reflects current usage status.
• Works with any carrier, any plan, and any billing cycle.
• Set usage alerts.
Four things that set us apart:
1. App Badge Monitoring-
• Monitor usage % as app badge without ever opening app again
2. Visual Monitoring-
• Theme colors will change to reflect your current usage status.
3. Accurate History, Export, and Charts-
• Keep your cellular providers in check with the ability to export accurate history of your data usage.
4. Today Widget-
•Monitor Data Usage like a pro with a widget that shows cellular data usage on notification screen under today.
#1 paid utility in UAE & Malaysia; top 10 paid utilities in Canada, U.K., Spain & Hong Kong.
"...to get into the nitty-gritty of your data usage you may want to splurge for Data Usage." 
-AllThingsD
Check out Data Usage Pro for custom usage counters.