iOS: If you've ever exceeded your iPhone's data allowance you know it can be expensive. To keep track of that usage, we like DataMan Next, and it's currently free.
DataMan Next is one of the prettiest data tracking apps out there. With it, you can set up alerts so you don't go over on your data plan, set custom thresholds, and even track your Wi-Fi usage. If you need a more granular view, DataMan Pro, which gives you info about which specific apps are eating your data, and maps your data usage geographically, is 50 per cent off today at $5.49. DataMan Next is offered for free quite regularly, but it's worth taking advantage if you can.
DataMan Next (Free) [iTunes App Store]
Dataman Pro ($5.49) [iTunes App Store]