A free app for Android, by AppWorld Infotech.

Free WiFi Internet Data Usage Monitor is a utility for monitoring Internet data usage on your mobile device. It will help you find out the exact amount of data that you are using on a daily basis, as well as how much of it is consumed by background apps.

This utility has a built-in mechanism for measuring Internet traffic, so you will not need to add any additional programs or plugins to your device. The app can monitor the usage of both Wi-Fi and cellular data, as well as work on any device.

The app is completely free, and there are no charges or subscription fees.