• Tracks the data transmitted over all active network interfaces.
• Notifies you about medium and high network usage through the Android notification facility.
• You can specify different transmission limits for every interface, so you can setup the amount of data that your carrier's price plan lets you transmit for free.
• You can schedule optional automatic resets on a daily or monthly basis.

Technical features:

• Does not depend on having a service running on your device, which is good if you care about your battery ;-)
• Automatically restarts after a system reboot (when you turn off your phone).
• Does not wake your phone up when it is sleeping.
• It's free!
• .. and opensourced, so you can browse through the code ;-)

For more information about the application's functionality go to the help page. You can find some screenshots here.