Johnny Ixe has DataMan Next Track Data Usage In Real-Time (iTunes Link) for the iPhone and iPod Touch in the iTunes App Store for FREE (regularly $2.99) for a limited time.

Your weapon against overage. In its most advanced form ever. DataMan makes it easy to track your data usage in real-time, with key statistics you need most in an incredibly clean interface. It also includes smart forecast that predicts if you’ll stay within your data cap.