Android

I was able to find one on the Android Market (Google Play) called Onavo, it is a free app that shows how much data is being used by an application and gives suggestions on what you should do to lower your data usage as well as allows you to set how much your plan is, (MRC and data allowance) as well as when your billing cycle starts. This could be used for customers..... I have only tested it for about 2 days now. So I don't know how accurate it is. But it seems to track it pretty well so far.

I also found one of Data Manager Free. In Google Play. It did the same thing and allowed you to create a pie graph showing how much was being used. It has an area where it tracks your mobile data vs. Wifi.... Only problem I saw with that one is it is kinda hard to use and under the usage where is shows the data it shows how much it is using of the CPU which may confuse customers. That is why I didn't recommend this first.