App to track Internet Data Usage !

I use 3G watchdog for precisely that purpose

 

Yes that's an app. Search for it in the Market.

 

Yep, I use it as well.
Does the job, loads of different settings to suite just about everyone.
I thiink there is a free and paid for version if memory serves right.

 

I use "My Data Manager". Its really simple and easy to use. Will notify you when you hit the limit you set. Also, has a widget and can be place in the notification bar. Displays each apps usage too. (I don't remember if its free or not.) But well worth the money.

 

I've also been using "My Data Manager" ever since I picked up my S2. And there is both a free version and a paid version, I'm using the free version.

 

Well, i 'm biased because I like 3g Watchdog, I'm also not quite smart when it comes to phone workings and the inards so to speak.
3G lets me set up weekly/monthly data use. set a start date and weekly/monthly use. The Appdoes the rest and tells me aas the month goes by how many days left, % of my data allocation used, and if I continue at the rate I'm going how much will be left at the end of the month - or when I will use it all.

It can run out of site and open from the drop down task bar, or icon on a home page - I've only used HTC desire and Sensation.
Also options to monitor data roaming as well if you travel.

That's all I know, but it does a lot more, you can have graphs, or whatever. I stick to simple words!!