I don't have an unlimited data plan so I need to keep an eye on my mobile data usage out of plan data is very expensive. This app lets you enter your data allowance and when it resets, then it just sits quietly in the background, if you pull down the notification drawer it appears as an ongoing task that shows how much data you have used and how much is left. You also have the icon in the top left which will change from green to orange and then red when you start running out of data. The app has a widget but I find the info in the notification drawer is all I really need. The figures in the app correspond pretty well with what's on my bill, it's been a little out lately but that's because I've been changing roms and restoring the app data from backups so not the apps fault. The unintrusive nature of the app is my favorite feature you can just set it and forget it. There are some additional features like a history of data usage for the last few days of months with the daily and monthly average all with options to export or import as CSV, useful for seeing if you need to upgrade to a higher plan. If you need to keep an eye on your data give it a go, it's free and isn't even ad supported.