You can add a beautiful and simple widget to your home screen.
You can customize the app to show the data usage for your billing cycle. Monthly, weekly, X days, manual reset, you name it.
See detailed data usage per app. You'll know which apps are eating away on your quota. Filter on Wi-Fi/mobile, download/upload and roaming.
Highly customizable:
More features:
For a small amount you can support the developer and unlock extra features: